If your tank was aiptsia-free I would move aggressively to get rid of the ones that you imported on the clams. I don't know if they nettle clams. I do know that they can walk, and if they do they leave a trail of babies behind them.
I have found two ways to effectively kill aiptasia. One is to remove the rock - in this case the clam - from the tank. With a moist toothpick, pick up a little clump of dry lime (calcium hydroxide, kalk, whatever you choose to call it). Scrub the powder into the anemone, making a paste of it and the dissolved lime. Let it sit for a couple of minutes, then return it to the tank. Be careful not to contact the dry lime or lime paste against the flesh of the clam, this will burn it. I've used this method to kill aiptasia off snails and it didn't hurt them. I've never done this to clams.
An effective way to kill aiptasia on something that you don't want to remove from the tank is to entomb the aiptasia under a glob of reef epoxy. This will leave and ugly lump of epoxy on the shell for a few months until it gets encrusted over enough to obscure it. I've never done this to a clam.
I don't think you would want to introduce a butterflyfish or other natural control unless your tank was overrun with these little brown buttholes. if you just have a few, IMO, go for manual extermination.
some people advocate using kalk paste inside the tank. I've tried this and don't like it. First, it doesn't seem to kill them permanently. Second, if you miss with the syringe and get paste on the clam, you'll burn it.
I don't claim to know much about clams, but i have been able to keep an aiptasia-free tank. don't let them get away from you. Good luck.
