A vermetid snail wouldn't really "burrow" it will just attach itself and start growing its tube outwards. 
 
In my experience (if this is what you have), they proliferate a tank rather quickly, and although the main consensus seems to be that they're harmless, I find the cobwebs to be extremely unappealing visually; and I would go so far as to say they're not "really" harmless because in my experience it can irritate a coral to death where there is contact (be it zoanthid, other softeys, LPS, and especially SPS. They did a LOT of damage to the corals in my tank and I no longer let them be when they show up next to a coral.) 
		
	
		
		
		
		
			
				__________________ 
				 -- Tony 
 My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills.  Whee! 
 
			 
		
		
		
		
	
		
		
	
	
	 |