Aiptasia on the inside of clam?

This is a tough situation. I'd advise against kalk or other "injectables" as they could hurt the clam. That leaves two other choices; mechanical removal* or adding something that will eat aiptasia (scats, peppermint shrimp or berghia sea slug).

First I'd like some clarification...
How "inside" are we talking? There shouldn't be any exposed shell on the inside of clam where the anemones could attach. If there is interior shell exposed then there is likely something else wrong with your clam. Could you post a picture or two so I can get a better idea of the situation?

Without knowing more here are my thoughts...
If the anemones are completely inside then mechanical removal is out of the question and adding something to eat them is your best bet. But if the clam closes, whatever you add to eat the anemones won't be able to get to them either... hmmm. But if this is the case (and the clam is in the condition I'm assuming it to be) I'm not sure how long your clam will survive even if you manage to rid it of anemones. So, I'm hoping you have the following situation.

The anemones are on the very edge of the clams shell and retreat into a growth ridge when the clam closes quickly (making it look like they're inside the clam). If this is the case then you could probably use a pin tip or something similar to scrape them out (attach the pin to the end of some airline and have it siphoning into a bucket to remove all anemone bits).

These are the two situations I envision from your description. If the situation is different please post a picture or two and provide some additional information and I'll try to come up with additional suggestions.

HTH,
Tom

* there are many people who believe you can't control aiptasia this way and that you'll encourage them to spread. If done improperly this is true. But IME if done carefully it is possible.

Here are some photos

Thanks for posting photos. Congrads on getting one out. I wouldn't worry too much about damaging the mantle. As long as the clam is healthy and the damage is minimal it should recover.

That being said... it looks like your clam is not growing (not healthy). I don't see a white band of new shell growth along the outer edge of the shell and the mantle is not fully extended (aiptasia will only impact mantle extension right around them). Lack of growth is usually a sign of either water parameters being off (Ca, alk, or Mg) or insufficient lighting (or both). During this time the clam will continue to add layers to the shell but the layers do not extend past the previous layers. Resulting in the ridges and groves at the edge of the shell. This is where i believe your aiptasia have taken hold.

The good news is that if you correct the issue(s) then, if strong enough, the clam will begin to grow normally again. I'm not suggesting the current condition of the clam is your fault as it may be a result from the previous owners care (or lack thereof). But I wanted to let you know so you can provide the correct environment and get this clam happy again!

HTH,
Tom