commensal acro crab
Chances are that the crab you have found will not be a problem. Crabs found in heads of SPS corals are usually of the Family Xanthidae. They are typically commensals which sometimes protect the coral from predatory fish or invertebrates. However, all of these crabs should be watched carefully as not all of the Xanthids are harmless - see below. Trapezia sp. (Family Trapeziidae) is usually found in Pocilloporadae while species such as Tetralia and Domecia (Family Xanthidae) are the typical residents of Acroporidae.(Delbeek and Sprung The Reef Aquarium, Vol. II, pg. 324). In the experience of the author and other reefkeepers that the author has conversed with, hairy brown crabs imported in SPS colonies tend to be harmful rather than commensal, while the tan, smooth shelled crabs like the one in the above picture are peaceful, well-behaved residents.