I have many of the stomatellas in my tank and never seen one graze a coral base. The things that will graze a healthy coral from time to time are some of the Chitons. The link above has a couple pictures (5th) down of one species, although maybe not of the one that would eat coral. I have a number of Chitons too, and only had one that ate coral. Coral flesh around the encrusting base of a colony may need to be eaten to keep it from getting RTN, but, those Chitons I have seen go over the top of my Poccolopora and grind it down smooth. They aren't really after the coral, they are just grazing on what happens to be there in the way as they looking for algaes. I do have something that comes out now and then, here and there and does a lot of damage. One night it ate half a goniopora, last night it put a clean dent in a large colony of yellow polyps. So much for not getting any nasty critters by using only dry aragonite rock.. Hitchhikers! The Chitons that I have are real close to the description of the ones you gave here. I guess they are a different species though as they are not bothering anything and live in the substrate during the daytime.