I saw a lot of those when I went to Fort Walton Beach, Florida (AFB air force base), it's a caribbean hermit crab (couldn't find a website to confirm). It was an inlet waterway (saw them on the rocks above the waterline, crawling near the shoreline, in and out of the water), there were a lot of algae patches in that area, not sure if they are reef safe. They ranged in size from smaller that a dime to as big as a baseball (maybe a little bit bigger). I would keep an eye on it to make sure it won't cause any harm.