This little guy seems to have come in most likely on some sea lettuce (but possibly Caulerpa prolifera) that I bought online and dumped into a quarantine tank. I started seeing sheds after a few weeks, and of course they got bigger, before I finally found the guy. His carapace maxed out at about a centimeter before I released him into the display tank, whereupon he would have been devoured instantly by my bicolored dottyback had I not tapped on the glass to frighten the fish away until the crab could descend into another macroalgae. That episode caught be by surprise because the dottyback never showed any interest in my emerald or hermit crabs. I guess I should have put him straight into the refugium. This little guy looks a lot like some kind of ~spider crab (Inachidae) to me - tiny, tiny claws and general body shape. These land photos don't convey how hairy it looks underwater. I can't imagine it could be harmful to anything since his claws are so small, but who knows how big it might grow if it doesn't get eaten first. Anyone have any ideas what it is or where it is native to?