I have another one on a different piece. A friend was telling me the hairy ones with blue eyes are bad. I noticed this one does have dead flesh where the crab always sits and blue eyes.
"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. "
I have had thrm before they have never just eatten one little spot and stayed there. They would start to eat as much as they can as fast as they can.
There are beneficial ones like the porcelain crab but there so small amd hard to find that no pet store sells them I wish they would tho.
Its up to u with what u do but I have never had luck with them in the past