[It looks like a red mithrax crab. I believe they are the same thing as emerald crabs, but red instead of green. I've got a few smaller ones and they all eat algae.]
[That's what it looks like to me, too. Just look for the red in the bend of the arm and that will tell you if it is an actual mithrax that eats algae like the emerald colored ones. (I believe I have that right...)]
