Peppermint shrimp do not generally eat coral. When first introduced to a tank they may take a bite of a soft polyp but will learn that it is not edible. They consume detritus, uneaten food, and decomposing organic material. It may sometimes look like they are eating coral while they are searching for food.
Camel shrimp look like peppermint shrimp and eat coral. Familiarize yourself with the difference and make sure you have peppermint shrimp. Peppermint shrimp are great for Aiptasia control, but unfortunately some go after it with a vengeance and others do not touch it. Keep trying until you get some winners.