I posted this picture up in reefcentral, and someone replied with:
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				It is definitely a Petrolisthes. Possibly a P. lamarckii. 
This guys very harmless filter feeder and very good for your tank and nice to watch them, if you see them. :-) I have some different Petrolishes in my nano tank. 
 
Here are two different crabs of my own Petrolishes.
			
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