its a porcelain crab id by its 3 legs on each side instead of the usual 4 like real crabs, they cool completely harmless, i have 2 that hitchhiked on my LR
I'd have to go with THEDLO, it looks like a green version of a porcelain crab. I never seen them in that color, but everything else about it looks like a porcelain crab.
I have tried to find this crab in my ID books and on-line ref but can't. So, all one can say is that it is a Porcelain as THEDLO stated. They have as stated only 6 walking legs, as the 4 set of walking legs is highly degenerated/reduced and held tight up against the abdomen.