bad crab?

Try to post a picture of it or at least a description.
People here can and will help out if you give enough information.
 
Probably Trapezia or Tetralia and therefore a beneficial commensal which helps groom & protect the coral. Colonies with these crabs are healthier & grow faster than corals without them.
 
pic

pic

here is a pic.
28516DSC02408.JPG
 
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