The residue could be any number of things, and possibly more than one of them simultaneously - precipitated minerals from the water (calcium, magnesium, etc.), bacterial growth (our indoor is far from sterile, and anything drifting onto wet surfaces can grow or breed), exudates from the plant itself (oozed sap, secretions, byproducts from root cells, etc.), or yes, possibly mold, the spores of which also waft around in our air and through open windows.

All you can do is to continually clean the container out and scrub-down the stones when needed, using dish soap and perhaps afterwards bleach that is thoroughly rinsed off before reuse. Ideally, the plants should be potted instead, as "water culture" does not suit the vast majority of houseplants, especially not long-term. Snake plant in particular needs fairly dry soil (between normal waterings) to thrive; as a near-succulent, they do not grow in wet habitats in the wild. Spider Plant and Pothos (perhaps your other plant?) are more tolerant, but even they will succumb to root death when they can't get enough oxygen. On plants which are not aquatic by nature, roots need a certain amount of oxygen access and gas exchange in the soil to stay healthy. Although how long it takes for root cells to start dying will vary from one plant to another, it tends to be inevitable if they're not moved to well-drained potting mix sooner or later. Water can be useful to encourage rooting for propagation (though it's not needed), but it's not the recommended substrate for long-term growth. It's also hard to provide nutrients to plants rooting in water, since fertilizers risk growing algae more than they will nourish the plant.

Miri