Black algae/fungus/mold? Help!

[Datreeferman]

Hi everybody, I've had a black or very dark green fungus, algae or mold on my drift wood for a while, I've heard that it was a bacteria build up and that I'd go away, so far it's been on my wood for a couple weeks and it hasn't gone away, about a couple days ago it started spreading to my plants so I thought it was algae but then just yesterday I noticed it on the back of my mollies tail! Black stuff on the edges of his fin! I'm not sure it's the same thing but it's gotten me worried. What is it? Is it harmful? How much? What should I do?Any help is greatly appreciated.View attachment 230616

 

[Datreeferman]

Sorry, I forgot to post that my params are good exept my nitrate is a tad high, I found out that my plants have what's called fuzz algae, but is that's what's on my driftwood too? It looks a little different. And what about my mollie? What's on him and how do I get rid of it?