Possibly a Xanthid (or Xeenid, not sure how its spelled), or common acro
crab. They commonly take up residence inside branching corals and have a harmless relationship. The coral provides protection, the crab wards off attackers to the coral. However if your not 100% sure its a Xeenid I wouldn't feel bad about pulling it.
Here's to swimmin with bowlegged women.
Reef with 30G Sump/Fuge Combo
-30G LTAnemonarium with mated porcelain crabs, 10 sexy shrimp, and 2 pink skunks all tied into the other 135G in my system.