I've got crabs! (What are they?)

[hillegom]

Not a very good pic.
What kind of pincers does the unknown crab have? If sharp, probably a meat eater, if like the emerald they are dull, then maybe he will eat algae.
But all crabs are opportunists. eventually they all are bad. Depends on how hungry they are.
You could catch him in a trap. put some food into a long glass, lay it with a slight angle on a rock. Hopefully the crab goes in and its too slippery to get out. Then you could take him down to the LFS to ID