I had a couple of them in my old system.  Never could get a 100% ID but the closest I could get was a common name rock anemone.  It didn't grow very much and never bothered anything.  At night it would strech out but again never hurt anything.  I fed it a couple of times. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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