Where did you get them from? there are shrimp that are called peppermint shrimp but it will not eat aptasia it is californica and like cooler waters and it is often never seen after added to the tank and tends to pick on corals. The peppermint shrimp you want is the wurdemanni, it is very hard to tell the difference. The store should be able to tell you what they have, my guess is that they do not know. Their legs will have no white or yellow stripes on them.
All the best.