Description:
Acro Crabs, as they are commonly called, are always found within the branches of SPS corals.  They are small (< 0.5" across) with a white body and a black line across the plane of the eyes giving it a bandit look.

Good or Bad?:
Acro crabs are harmless commensals, apparently living in harmony with their associated coral.  It is unknown what they feed on, but presumably the mucus secreted by the coral or perhaps particles trapped by the coral polyps.  One of the thumbnails shows a crab eating the detritus captured by a large tube worm mucus net which is near its coral head. They will also eat meaty food that falls into the coral, but they do not need to be specially fed.  They will die if separated from their coral and should be left where they are found.  They will sometimes migrate from coral to coral, but usually stay within the same coral colony.

Notes:
There are other crabs that may be found in the branches of SPS corals including a small green hairy crab.  These have been observed eating the polyps of the coral and should be removed.  The Acro crab is easily identified by the picture above.