if possible the best way to get rid of this especially if you want to keep the rock is to buy or make an anemone box or similar closed container. Buy your peppermint shrimp and contain them inside this box (preferable location is in a sump but can be display if tank size is limited. Put the rock or coral with aptasia in the box for few hours or until the aptasia is gone. Peppermints can go after a myriad of things in the display and even eat stressed corals. By isolating them with your target rock or coral they have no choice but to eat what is in that box with them. Aptasia gone, rocks and corals saved, repeat as necessary until your tank is clean of it