It's called hair algae. Every tank gets it sooner or later. Urchins eat it, sea hares eat it, and some fish [lawnmower blenny, tangs] eat it. If you're feeding flake or pellet food, that contains phosphate that acts as fertilizer. You can try Phosban [read instructions carefully] or various other preparations. Ther e is some indication Phosban used too long may harm zooxanthellae.