I had the same thing. ( I have 9 clams), anyway I noticed that on 3 of them that the mantel across the organs was basically GONE!! But the clams still opened and closed and the matel on the outer edges extended. Thought they were dead for sure. That was about 2 months ago. They still open and close.
But I was having a hell of a time with my calcium levels, corrected now. They appear to be, for a better word, repairing themselves? I hope so at least.
Anyway, clams are funny....I also have been checking them more and more..found some red bristleworms hanginging around..