wcroft, you say you found him? well i dont live in the keys, but when i first started i was living in WPB, and i went to peanut island one weekend and grabbed a bunch of snails, microhermits and other critters i found in the rocks, one such critter was a small little crab that looked like what you have there. he was cute so i figured all would be well.
i dumped him in the tank, and moved down to FTL without ever seeing him again.
i noticed lots of snail shells starting to pile up increasingly over the months. then a missing blenny, and missing firefish and missing shrimp.
then i caught a glimpse of the beast that i dropped in there, easily quadrupled in size. it took weeks for me to catch him. when i finally did he was the size of the inside of my hand. unfortunately it was also when Wilma struck, and the LFS was closed, so i could not donate him. into the bowl he went.
he was a florida stone crab. the pic below is about 3 months before i got him out, and much smaller.
side note: there are many thousands of known and as yet unknown or un-named crab species. they are one of the hardest critters to categorize and tell apart. yours could be any one of them. from hobbyist experiences, we have what seems like just about all of them decimate our tanks except for a spare few. the claws are a great tell as to what the crab uses them for. most crabs are scavengers and hunters, and will eat anything they can grab hold of.