the little brown fuzzy guy with blue eyes in Joels first two pics will munch on the coral tissue and polyps. If the acro is large enough and the crab moves around, the damage will heal. In that case I still consider them a benificial crab, because they will still chase away some of the predatores we have been encountering (AEFWs). However, if the acro is small and the dammage is localized, and the crab doesnt move on, the coral will definately suffer from the constant munching. Moving the crab to a large head or to a trigger tank is a good option.