I just saw this guy come out of a cave in some live rock and snatch a snail and pull it out of its shell and eat it. I caught it and put it in this plastic cup to take a pic...What is it??? It has 4 furry legs on each side and 2 olive claws.
I concur with the previous two statements. It is an Emerald crab or Mithrax crab. It eats bubble algae and other algaes. As for what you saw, I have had them in my tanks for years and never ever saw one of them pick on any of my snails.
Hold the phone here kids. I have several emerald looking crabs that are killers and not emerald's. Here is how you tell. Look at the tip if the claws. If they are sharp points, KILL IT NOW!!! If it has flat pads on the end, throw it back into the tank. When I remodeled my tank I found 5 emeralds hiding in holes in the rock. 4 were not emeralds!! They were killers.
I have 2 of these in my main tank, one is now reaching 4" in width. It makes a habit of eating my snails and piling the shells near its holes. I have yet to catch the bugger. I say take it to the LFS and let them feed it to any octos or triggers.
I too have a big crab lurking in the rock and can't get him out. Mine likes to smash the poor little hermits against the rocks until they submit. No luck with the glass trick yet. Hope I don't have to tear the tank down like Wade did....