Black Mithrax Crab

[antlers]

Found this in my live rock, looks identical to the one in this picture:

He only comes out at night.

My question is whether I can keep him in a tank with fish and live rock only (no corals) or if I should remove him and place him in a separate aquarium. I feel bad isolating him but he is currently sulking in a breeder box in my tank because I'm afraid he may nip at my clowns, who like to sleep on the sand at night. I've read that crabs like to eat whatever they can grab.

Not sure what to do. He's pretty cool looking. Anyone know how big they can grow? He looks like he's about 1 1/2" - 2" right now.

Thanks

 

[GerritM]

Hi!
Not a Mithrax (Mithraculus).
That´s a Chlorodiella sp.. A good algae eater. Relativ harmless.

here ist my Chlorodiella cytherea:
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[antlers]

any idea how big he'll get?