Cymo andreossyi : Very common and is considered a coral symbiont, with the caveat that such symbiosis are a trade off between the coral and the crab, the coral gains protection and house cleaning services while giving up a few polyps, mucus and captured/settled food particles in exhange for those services. A healthy coral should have no problem repairing or replacing lost polyps, but again, in an aquarium environment, an eye should be kept on the coral for excessive damage being done due to the coral being unable to recover as fast or faster than the damage being done by the crab. In short, its a judgement call that you will have to make.