Not much you can do. It's normal, just one of those things that happens. It will run its course but it could take a while. In the meantime you could wipe it off and siphon it out, but it will likely come back. You can also pull it out, rinse it with some hydrogen peroxide and soak it in water and Prime but again it will come back until it runs it's course. It will go way eventually.
You can also get a snail, who would eat it happily. Mystery or nerites, both like this kind of thing and will keep wood very clean. Nerites also lay eggs on wood, and some do not like the look of the eggs. They don't hatch, they need salt water for that. But they are hard working, efficient little clean up crew members.