I am not sure what the "bug" is, but from what Barry has hinted toward, it could be something that stays with the clam, and gets out of control sometimes. Just like most of our tanks have the potential for ich occurrances, but it takes a stressor for them to affect the fish, then they multiply out of control and seemingly healthy fish are also affected.
I don't know, because i'm not a marine biologist, and my reading on the subject is rather limited. But, lets entertain for a moment that all clams have this "bug", but if you add another clam, that is stressed, that clam may be prone to attack and the bug may breed out of control and affect other clams.
Now, please, take everything i'm saying as absolute assumptions, I have no evidence of any of this. I am entertaining this idea and i'm treating it like this is the cause right now. I am trying to remove the iron in case that is the food the bug indeed needs to survive, and i'm trying to make the water very clean and improve the chances by removing all stressors.
If I see no further improvement then I will dip, but if they improve to where I see no signs of stress in the form of pinched mantles, i'll assume that the parasite is still present, but the numbers are under control and the clams are able to deal fine.
Time will tell, obviously a lot more research needs to be done here to give definitive answers on this. I have read that the FW dip works, and i'll try that if I don't get results with what i'm doing.
thanks,
Angela