Mystery Crab ID Please

[Tetra1]

Can anyone tell me what type of crab this is? I found him a year ago and at the time he was tiny. He came in on my ulva lettuce. I have since broke down the old aquarium and set up a new one and would like to know if he is friend or foe? Thanks.

 

[mariobrothersleeve]

Looks like an emerald from the picture, but the claw looks like a predator crab.

 

[fastrd400]

How big is he?

 

[RockStacker]

I don't think that is an emerald crab.
Emerald (mithrax) crabs have that distinct spoon-like shape at the very tip of their claws.

That crab has slender and sharp claws. I would not add it to the tank until you have observed it to be a herbivore/scavenger only.