Is that near the compressor or where they worked on the lines as it'll likely be compressor oil @HungryHippos, when you release refrigerant to connect on to the service valve a small amount of oil in the lines also escapes.
I would try a degreaser on it.
	
		
			
		
		
	
				
			I would try a degreaser on it.
	
 
 but if it's obscured from view like yours is then should be alright.