Hmm, I suppose they could do that. Never seen any eat a coral. Maybe they were eating what was eating the coral. Red bugs,tegastes or something else. Hope you are right and the problem is solved. Points out the need to look around at night once in awhile. Surprises lurk in the dark.
I do get a little recession here and there on some corals from time to time but not much and not often anymore. I'll dip em an/ or frag em when I do. Sometimes it just keeps moving along though. Don't know what causes it. I suspect predation, allelopathy, alk shifts, localized hypoxia at night,and and an endless list of other possibilities.
Sorry , you are having this trouble.