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Hard to say for sure from the pic, but possibly the Hairy Coral Crab, Cymo andreossyi?
If so, they're somewhat parasitic on their host corals; in small numbers on big colonies, they're helpful, but in large numbers and/or on small colonies, they may kill the colony.
Probably Cymo andreossyi - generally not one you'd want in an aquarium, but at least somewhat beneficial in the wild:
https://aquadocs.org/bitstream/handle/1834/9351/Cymo andreossyi.pdf?sequence=1&isAllowed=y
Looks like that to me too but @ISpeakForTheSeas is probably rightI am leaning more toward Decorator Crab....