they are usually flat crabs to move in and around in the
acro colony. Some times red, sometimes white, i'm sure there are lots of different kinds. They will primarily stay in one particular acro but occasionally will move to another colony for a better home. I've never seen one on the rocks. Generally, if the crab is really hair looking legs probably not an acro crab. Most crabs will be harmless to your corals, if large enough they may pinch at fish but that is usually rare too.
If you have acro crabs and want to keep them, quarantining all acros will be a must. If you end up with red bugs in the tank, the treatments will kill the acro crabs so need to use best preventive means possible to keep from get from having to treat your whole tank.