Looks like one of the beard algae that people treat with Hydrogen Peroxide.. I had black tuff algae (I think the same as yours, at least with treatment) and I just did my best living with it till I broke the tank down. I got it from doing too many large scale water changes straight from the tap. The tap water had high co2 and the tank had very little since it was low tech and super low light. Now I keep a 5 gallon bucket in my utility room with room temp water and top off with that to let the co2 gas off.