Those are mysid shrimps and are very common in the display tank when placed with live rocks ( I think they came along with the LR ). However, their life cycle is short, may be less than a couple of weeks. If not being eaten by the predators like wrasse, they can regenerate quite quickly and steady. 
A perfect live food for the seahorses and pipefish other than amphipods 
		
	
		
		
		
		
		
	
		
		
	
	
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