Crabs that look like ?

[Doug]

Good title. I bought a couple pieces of new rock for my tank. Have them in a bucket of saltwater. They have been in a holding tank for a few months now.

So I treated the rock with exit the other day and was looking it over last night. And there was a crab sitting there looking at me. Identical to a emerald. Scurried back into its hole before I could do anything about it. Left the rock out of the water for a few hrs. to see if it would come back out but that was futile.

I,m searching as we speak but wondered if any similar to emeralds exist that should not be in out tanks?
Thanks