I know exactly what you're referring to. My green emerald had these and they are so fast. They remind me of electrons orbiting nucleus. 
 
When the crab would shed I noticed the bugs were gone for a while then would slowly return. They didn't seem to be harmful. Mine were orange. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
	 |