They'll strip it to the bone JL. That particular tank is full of java moss, but if you look closely at it, all of the little leaves have been eaten off the stems. It doesn't seem to affect growth too much, but it's not terribly attractive. Something to look out for, but amano shrimp will do the same thing.
http://www.aquamoss.net/Articles/Siamese-Algae-Eater-And-Aquatic-Moss.htm
http://www.aquamoss.net/Articles/Siamese-Algae-Eater-And-Aquatic-Moss.htm