I watched a large emerald crab snap off a tip on an SPS in cali's old 150g SPS tank. Just watched in amazement as he was munching and kind of had a look of, so what?
ya I have had them eat some sps, after about the 4th time I took them all out. I don't know if it's the lack of whatever or just the nature of some.?
I have also seen this happen at at lfs by me.
They sometimes do this when there isn't enough food in the tank. They are eating the algae growing inside the corals. Try feeding them with some nori and see if this stops.