The weed in question is Woodsorrel (Oxalis), a common volunteer in gardens and outdoor potted plants in our area. There are several species that occur here, with some being native and others not; they can be challenging to tell apart. Fortunately, plants are easy to remove, though as with any weed, seeds may recolonize the soil at some point and return in future seasons. (Especially if it keeps blooming and developing seeds, since Woodsorrel seed pods burst explosively and fling seeds in every direction.) Where surrounding plantings allow, refreshing a mulch layer so it is about 2 to 3" deep will help discourage future weed seed germination.
https://extension.umd.edu/resource/oxalis-yellow-woodsorrel-weeds

The English Ivy the Woodsorrel happens to be growing in is, ironically, a more aggressive and troublesome weed, especially when it escapes into wild areas. If you are open to using alternative groundcovers, we suggest removing it entirely.
https://extension.umd.edu/resource/english-ivy

The black coating on the Holly foliage is called sooty mold, and while alarming in appearance, doesn't directly harm the plant as it is not a plant pathogen. Over time, though, its blocking of some light from the leaves might stress a plant, especially if the mold population is high. The reason for its appearance is due to the presence of a sap-sucking pest producing honeydew. Honeydew is an excreted waste liquid that is essentially sugar-water, and is commonly a result of scale insects in particular when it appears on holly. Once the honeydew-producing pest is under control, the sooty mold will have no new honeydew residue to grow on and it will eventually go away and weather off, though that may take awhile as the dried honeydew glues it to the leaf surface fairly well.

Look on the undersides of holly foliage, as well as on young stems/branches, and you will likely find the sedentary white or brown scale insects. Low to moderate populations are well-tolerated by the plants and don't necessitate treatment, especially if beneficial insects are already hunting them. For  more heavily-infested plants that are taller than a person and thus hard to spray thoroughly, consider hiring a professional pesticide applicator (often through a landscaping company) to treat the tree with a systemic product, as this will be a more effective and efficient control tactic. Dead scale may not fall off of the plant right away, but you should see a reduction (gradual though it may be) in sooty mold as their population wanes.
https://extension.umd.edu/resource/soft-scales-trees-and-shrubs
https://extension.umd.edu/resource/cottony-camellia-scale-shrubs

Miri