Ghost Crab? Fiddler Crab? Ocypodidae, in any case. The guide called them Ghost Crabs (Ocypode quadrata), but the claw size looks more like the Fiddler Crab (Uca pugilator) pics I'm seeing. Exposed around Red Mangrove roots at low tide. Didn't see these until a guide pointed them out -- the only visible part was that they they were waving their light-colored claws back and forth.