Copper Band butterfly fish are known to eat aptasia, problem being once the aptasia is gone they may starve as they are finicky eaters. The Matted filefish is also one to eat aptasia, but they will likely go after other corals, however I have heard of people using the filefish to rid the tank of aptasia and then removing it as filefish move relatively slow.
The preferred option to get some peppermint shrimp to eat the pest, as some will eat aptasia. And adding a few shrimp is not going to create any unwanted problems.
I am not 100% sure on any of my suggestions as I am not confident in what pest you have, these are suggestions for a treatment of aptasia that may carry over to whatever pest you have.