Can't be 100% but that looks like the algae I get in one of my tanks which I believe is a short form of black brush algae. Is it kinda fuzzy almost furry? If it is only on the decor (where BBA likes to live) and it's appearance bothers you, then you can take the decor out, spray it with some hydrogen peroxide, let it sit for a min, rinse it off and toss it back into the tank. This will not stop further outbreaks though. For that you need to figure out your imbalance with light, co2 and nutrients.
