If it is just a few, berghia nudibranch are difficult to use to remove aptasia. They can be effective provided you purchase enough and have enough aptasia to sustain them long enough to get the population up to remove all of the aptasia. They work really well in a control situation where the tank is only the affected rock with no fish. Fish feed in the nudi's. Manual removal is a great option as well as a proper peppermint shrimp if you can find one.