black brush/beard algae.
kill it with fire.
..in all seriousness though, I don't think it's harmful, but like nothing eats it so it can take over I guess? most likely from too much light/too much dissolved organics in the water column. soooo... iunno, decrease lighting, do water change and add floating plants?
....and wait for someone to respond here about how to get rid of it. (something about squirting it with excel I think?)
but the main deal is to fix the cause, not the symptom. I'm sure someone more knowledgeable will give a better answer. ^_^;