Probably just wishful thinking on my part but if a crab, in this instance an Emerald Crab, sheds his skin could it be mistaken for the crab?
My Emerald Crab has been in my tank for 18 days and whilst he's never been overly active he has seemed fine. Now however he's just sitting on the sand, rather than clinging to the rock work like he normally is. I also can't see any part of him moving, so my suspicion is is he's no longer with us.
I know crabs shed their skin to allow them to grow, so could it be what I'm seeing is his old skin and he's actually ok? As I stated in my first paragraph it's probably just wishful thinking on my part, but one can always hope.