Where there's one, there's likely more. There are a number of commercially available products, but a simple kalk slurry is (IMO) most effective. As far as control goes, the holy trinity are peppermint shrimp, matted filefish, and berghia nudibranch. Copperbanded butterflies to an extent as-well, but are tricky fish and most aquarists don't have a good track record with keeping them. Same can be said, actually, for the matted filefish. Not that they're tricky, but more that they can develop a taste for coral. Berghia can be expensive, and if there isn't enough aiptasia in your system, they will starve to death.
Best bet is to nuke it with a kalk slurry, keep an eye out for more, and invest in a peppermint shrimp or three (lysmata wurdemanni).