I just lost a clam a few weeks ago and this one has been having issues with the mantle retracting. The foot detached and I am able to see underneath and it’s got a visible cavity.
Reading into pinched mantle disease and trying a freshwater dip as a last ditch effort. Fingers crossed.
Clam should be full of tissue inside the shell. What you have is a clam that was stressed for a long time and lost a lot of mass already. The sell show no new growth at all. I fon't think he will do well.