Depends. If yours is a wild-collected and colony-sized (A. tenuis) its probabaly a good old acro crab which is quite safe and actually beneficial. Bottomline keep an eye on it and if you notice any bare patches developing in the interior of the coral, get it out fast. I suspect its an acro crab though because with the destructive ones you'd have seen some damage already. Also, if it has blue eyes Id pull it immediately.