Get a small, stiff bristled brush, take it out and give it a scrub. Algae will always survive and return, but you can work on the causes in the meantime. The algae is probably getting nutrition from the wood, so it'll be a process.
Be careful with the algicide. It's a dangerous product to play with. I won't let it near the tanks.
No aquarium can ever be balanced, but you can outlast problems. There are no quick fixes. I have a strange version of black brush that has popped out in a couple of tanks. It's diminishing now, after a few months of it being really annoying. Whatever nutrient it stumbled onto, it has reduced its supply and is now suffering from poor nutrition. That combined with my manual removal is finally paying off. But I knew I was in for weeks if not months.
I could have radically reduced the decor, removed all the wood for good and maybe succeeded more quickly. But I wanted the wood, so I played a long game with it.
Me and my algae - we come to understand each other. When the black clump stuff returns to the depths it came from, some other form will take its place. Hopefully, it'll be an easier one to manage around.