I have observed them eating coraline algae. Initially it bothered me, but I have too much now anyway, so...whatever.
But yesterday I came home to find my frogspawn on the sand. I think the blenny had been nipping at algae and knocked it off. When I picked it up, 2 of the branches broke off. I checked them out, and they were very very thin at the base of the old branches. I think the asterina stars may have munched on the calcium carbonate from the old dead part of the stocks. It didn't hurt the actual coral, it just fragged it for me. I'm actually gonna post this 3 head frag now, so I guess a thank you may be in order?
But this short story is just to say, I have seen them eat coraline algae and I think they will eat the stony dead part of some corals.