beard algae

Right now I have Moneywart, corkscrew val, and Anacharis growing in the tank. Over the past little while I've developed ugly dark fuzzy algae (beard??)growing on the leaves of mainly the moneywart. I try to clip these pieces away but I can't seem to get rid of it. Is there a better way, and what causes this type of algae to grown. Its impossible to wipe away, it goes right throught the leaves.