i suspected such ... but didnt want to make a fool of myself.. again.. lol ... whats bothering it.. is that it has no roots considering the plant size ... the root mass should be near as big as the plant above ... see link perhaps there is an aquatics forum... or a pond forum .. this is a very common plant to float in a small man made liner pond ... in MI anyway ... ken Here is a link that might be useful: link

Is it an artificial aquarium plant? Is it supposed to be a single plant or a clump of plants? There appear to be 3 different leaf shapes there.

I may have found the answer on this one - hydrangea macrophylla "Nigra", a.k.a. black stem hydrangea. Not positive about the ID, but it looks close.

I confess that I'm having a brain freeze about the identity of your plant, but the "disease" is black sooty mold, a 100% sign that there are insects present. The mold feeds on the sugary excrement produced by certain scale insects and whitefly, predominantly. It's not affecting the plant at all, other than coating the leaves, blocking photosynthesis. Locate and ID the insect, resolve the infestation, and the black sooty mold will go away on its own. What's your general location?