I vote crabs, if it was STN there wouldn't be any "islands" of tissue left, it tends to recede in complete sections. Get those crabs out of there the next time you see them. 
  
Alternatively, move the coral to a different location or find a frag rack to set it on where the crabs can't reach them.  However, its likely they'll find another one to pick at. 
		
	
		
		
		
		
			
				__________________ 
				Christy's Reef Blog
 
My 180 Build
 
Every electronic component is shipped with smoke stored deep inside.... only a real genius can find a way to set it free. 
			 
		
		
		
		
	
		
		
	
	
	 |