A forum where you may post your pictures and descriptions about flora and fauna you find in your tank. Find answers to the eternal questions of "What the heck was THAT?" and "What just bit me?"
That is probably his old shell that he shed. He's probably still alive. Most crabs eventually cause problems. Watch for trouble and try to catch him if he misbehaves.
Picture ain't too good,but it could be a pea crab or it's molt. Any crab should be watched closely. Even emerald crabs attack fish and inverts from time to time.