Both sphagnum moss and bark can work well for potting orchids, but each comes with slightly different care regimes, as the two materials retain water differently. As bark breaks down due to "soil" microbes, it might use-up some of the nitrogen being applied as fertilizer, but that's a fairly minor concern and as long as it's being repotted often enough that broken-down bark is avoided so the mix stays airy, it's not really an issue. Sphagnum moss can become more acidic with age, but that too is easily avoided by routine repotting (say, every 1-2 years, more or less). Orchid roots require high oxygen levels, which is why they are grown in potting mixes that don't use soil. (In the wild, they would mingle with moss and organic debris on tree trunks, exposed to humid air and absorbing moisture and nutrients from rain trickling down the bark.)

The orchid pictured looks like it has been chronically dehydrated. Bark may require more frequent watering than sphagnum, the latter of which is best allowed to get fairly crunchy-dry between waterings to avoid root rot. When any orchid mix is watered, it should be thoroughly soaked with a good volume of water so the excess runs out of the drain holes easily and carries excess nutrients or minerals (like in hard water) with it so they don't build up as quickly in the root zone. Orchid roots are very sensitive to over-fertilization, as they are adapted to low-nutrient environments and fairly pure water (rainwater).

Plants can be switched from one media to another (like bark to sphagnum, or vice-versa), but an adjustment in watering and fertilizing might need to accompany that. You sometimes have to just see how a plant responds, and orchids are slow growers.

Ice cubes are inappropriate for watering orchids, so it's good that you aren't using any. Watering when dry is good for many orchids, but this frequency can be impacting by ambient humidity level (dry air will require more regular watering), lighting (plants in low light will use less water than plants in brighter light), and potting mix type. Pots with larger drain holes, like orchid pots, will also dry out faster. (This is fine, but something to take into consideration as the moisture from a watering won't last as long as it would for a plant in a regular plastic pot.) Sphagnum moss is a denser material than bark and that can retain a bit of humidity around roots as the moss dries compared to the more open spaces between bark chunks.

Orchids getting too dry may need a soaking to rehydrate when a typical watering isn't enough. You can try soaking the plant's root mass (bark and all) the next few times it needs water to see if it can regain more firmness in the roots and leaves so they are less shriveled. How long to soak we can't say, but try around 20 minutes as an educated guess. Use tepid water, and empty the saucer of collected water as it drains. Use plain water for the soak, not fertilizer-water. Keep the crown (where leaves attach to the stem) dry or make sure it's dabbed-dry with a paper towel or something after you shake off any collected water when it's done soaking. Water pooling in the crown can rot the stem and kill the plant.

Orchids usually need less fertilizer than people may realize. One adage that is useful for fertilizing orchids is "weakly, weekly," though like most houseplants, this can be skipped entirely during winter since growth is naturally slower then unless plants are growing under grow lights on a consistent lighting schedule. "Weakly" refers to a dilute fertilizer solution (potentially less than what the product label advises, even if it's a product formulated specifically for orchids...maybe a quarter strength). "Weekly" refers to fertilizing once a week with this solution and the rest of the time watering is needed in the meantime with plain water. Granted, the fertilization frequency could be less than this. Plain water is good to flush out any nutrient buildup so the potting mix doesn't break down too quickly and so roots aren't exposed to those residues as they build up over time.

Why your other Phalaenopsis are faring well in the same conditions is hard to judge, but it may simply be that this plant became too dehydrated at some point and hasn't been able to catch up in hydration.

Aerial roots -- those growing out into the air and not down into the media -- are quite normal for orchids. Orchids can generally be categorized as growing in one of two ways: upwards (monopodial) or creeping sideways (sympodial). Phalaenopsis are monopodial, getting taller on the main stem over time. The lowermost, oldest parts of the stem and the roots attached to it naturally die off gradually as the top develops new roots and leaves. When a plant is repotted, if some of the lower stem and roots has died back, trim them off, though the aerial roots near the top of the plant don't necessarily have to be forced back into the pot when the plant is repotted. (If you're cutting off dead stem, though, attempting to do that will be easier since it will sit in the pot more deeply that way.) If you do cover the younger roots, be sure to not smother them with a dense application of moss or bark; they need enough airflow to "breathe" like the roots below. This is less critical than trying to rehydrate the plant through soaking, though. If it regains a turgid appearance with rounder, firm roots and firmer leaves, then watering can adjust back to how you water the other plants, with a shower or stream of water over a sink that thoroughly re-wets all of the roots and potting mix.

It's probably hard to increase humidity too much in the average home (outside of keeping plants in a terrarium with poor airflow), so feel free to try boosting the humidity using a room humidifier if you want. Orchids do appreciate humidity higher than the typical home has in winter here. Don't mist the foliage -- this does little to raise humidity for any meaningful length of time, and wetting the leaves can increase the risk of infections. Pebble trays similarly don't benefit plants enough to be worth the hassle of keeping the pebble layer clean of algae and bacteria. Instead, a humidifier, or potentially grouping lots of plants together, tends to be more effective.

If the plant sits where it is pictured in terms of its light level, we would not suggest lowering the light further. Phalaenopsis may tolerate low light levels for a time, but they do not thrive in that; they need either brighter filtered light (like through a sheer curtain) closer to a sunny window, or placed near a window receiving indirect light, like a north-facing or east-facing window. Overhead household lighting, like ceiling lights, don't provide enough light to sustain plants, and window light falls off dramatically the further you get from the window. (Even though, to our eyes, this change is hard to detect.)

Miri

Misting the roots won't add much moisture to the overall hydration of the plant, but in some cases probably won't hurt if it isn't getting the crown too wet in the process. (We don't see any signs of rot in the initial photos.) Since it's in combination with thorough watering, it's less of an issue than if it were the only way you were watering. Repotting is more urgent when roots have begun to rot due to staying too wet or oxygen-deprived; since the opposite is likely the case here, repotting can wait if you prefer to have the flowers finish first. Orchids can be repotted while in bloom, but logistically, trying that might make accidentally breaking the flower spike off easier to do, hence it's best to wait if the roots aren't in desperate need of attention.

Incidentally, the potting mix doesn't have to be all moss or all bark; it can be a mix of the two, plus any other amendments that are commonly used (if you wish) like chunky perlite, horticultural charcoal, or coconut husk chips. There is no one "correct" potting medium, as nursery growers and home gardeners can make any of them work with the right watering schedule based on their unique growing conditions. The goal is to have a somewhat moisture-retentive mix that is also very well-aerated/porous. Orchids grown in, say, humid greenhouses that are watered every day or two would do better in a mix that isn't very moisture-retentive, versus plants in a drier home setting that may benefit from a slightly more moisture-retentive mix to tide them over between waterings.

Miri