That's a bandit
acro crab, either Tetralia nigrolineata or Tetralia rubridactyla. These crabs are obligate commensals that won't survive if theyre removed from the host
Acropora. They get shelter & food from the
acro. In return, they keep the
acro clean & defend it from predators. Scientific studies have shown that
acros with crabs live longer, grow faster, & in general are healthier than
acros without crabs.
icon under their name to add to their reputation. 
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