I know what you're talking about. I've seen them on used Mag pumps I've bought and things. I have them on my refugium walls, my tank is quite new and I just noticed them after this post. They are really tiny white spiral filter feeders. Completely harmless im sure. 
 
Although to an addition to this post. I actually have baby snails in my refugium. They came with some LS. I put one in the tank to see if it would grow. Not sure what  kind they are but they aren't the little rice shape clam eating ones. They are round. I have no pic, but christy might. Anyone have these? Do they actually grow into big snails? 
		
	
		
		
		
		
		
	
		
		
	
	
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