Are you POSITIVE they are eating the coral and not just picking at it? I have 3 in a 14G nano with a ton of various corals. They are always walking around picking at various things and even when I starve the tank for 3 days they dont touch my zooa's, lps or sps.
One thing I have noticed though, is if I put in some cyclopeeze or other fina particled food they will LOOK like they are picking at my corals, but all they are doing is picking the tiny itty bitty particles from off the coral. I've seen what looked l ike one eating a polyp many times only to see the coral untouched after the sexy wanders to another spot.
I think they get a bad rap because of what it looks like. Just like how bristleworms get blamed for killing stuff when all they are doing is eating decaying matter.
But give them a sinking pellet each if you think they are not getting enough food and being driven to taste coral.