I have the same filter feeder crab in one of my tanks. I believe it is harmless, probably some sort or porcelain crab.  
As for that crab , I have no idea, but you know what they say: if it has sharp pointy claws its bad. 
		
	
		
		
		
		
			
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				If it is just us, 
It seems like an awful waste of space.
			 
		
		
		
		
	
		
		
	
	
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