Chelmons eat worms, not polyps or anemones. I had several of them and they never went after anything but worms (killed all my feather duster worms). They never even looked at polyps or the expanded Aptasia in the tank. They only nipped at some of the retracted Aptasia whose limp tentacles they may have thought to be worms, though that hardly affected the Aptasia population in the tank.
The fish of choice might be a Chaetodon kleinii - at least that was the one everyone in Germany used to get Aptasia under control, but I never tried it mayself.
The best bet are IMO peppermint shrimp, especially if you have mainly SPS (they may harass LPS and anemones by trying to steal their food). Peppermint shrimp have the advantage that they can survive without Aptasia and therefore prevent a flare-up when some are introduced with new rocks or corals. Also they are easy to get and cheap.
If you get two or more they will also regularly spawn and produce plankton food for your LPS (the LPS's revenge for the harrassment).
Berghia nudibranches are the safest bet as they eat nothing but Aptasia. But for exactely this reason they will starve to death as soon as they have taken out the last Aptasia and your tank will be unprotected again. They are also a bit more difficult to get and cost more than peppermint shrimp.
