I would clean off the shell at least and inspect it. Does the edge show any growth? That is really important to be objective with that and look for real growth since you have had it. Clams don't really show much decline or sickness until they die, but stop growing. It's very possible it wasn't in good shape when you got it and is now on its last legs, so to speak.
IME it will die as is since it's been going downhill anyway so a FW wouldn't hurt, but I fear it might be a goner, keep us updated!
Not much is known about clam issues and disease compared with fish. I never really did much to qt clams, but I had one I did a short qt with and it introduced a mystery disease and killed the other clams - not PM or anything like that, but I tried everything to stop it and nothing worked. This was a long time ago, but now if I add any new clams they get qt until I see new growth and am 100% sure they are growing.
EDIT: looking more at the pic that doesn't look like PM to me personally, and I don't see growth. I think the poor guy has just been going downhill. Do the other clams in there have nice fresh growth on the shells?