I think I know what Talmier is talking about - I have something similar living in my candy cane. Between a number of the coral heads there are small holes, approximately 3mm diameter, in which something is living and feeds at night. I actually think it is gradually killing the coral head by burrowing into the skeleton of the coral. I tried to take a photo a few weeks ago but not very successfully. I don't think it is a crab but more likely a worm of some sort??
Does this sound similar Talmier?