Peppermint shrimp as a method of aptasia control is sorta hit or miss. It worked for me, but you have to be patient, and the aptasia have to be pretty small still. If the whole thing is bigger than the main body of the shrimp it'll never eat it. Wait a week or two, or try the boiling water trick on a few while you wait for the shrimp to get the others.
You could always take the rock out and give it a freshwater (RO/DI) dip, if it's accessible. Still, I'd wait to see if the shrimp go to town. Just make sure when they're all gone that you take that into account with your feeding so that the shrimp doesn't starve.