i have a Lawnmower/Algae Blenny (Salarias fasciatus) that has been with me almost the entire life of my tank.
i've never noticed any damage to any of the corals (LPS, SPS, or softies) that i could attribute to him. he's usually just fine picking at the rocks, eating algae off the clip, laying around, or snagging whatever i feed.
i have noticed what look like his mouth marks on the mantel of my crocea clam from time to time, but if it is him, it doesn't appear to be causing any kind of actual damage or discomfort to the clam. the mantle is always well extended, shell growth is excellent, and the marks usually go away after a few days.