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[mallyk]

He hasn't bothered any corals yet, but I probably will end up keeping him in my sump. He's kinda cute!

So no one knows what he is?

 

[graveyardworm]

Its a crab is probably about as close as you'll get for an id. Specific id of a crab is very difficult especially from pictures. Like most crabs it is an opportunistic feeder and will munch on whatever looks good at the time. I would remove him and give him a home away from the display like in a sump area where it wont get sucked into a pump intake.