I am sorry to hear that.
How are the test results from the MH tank? No ammonia or anything right?
The only thing I can think is maybe it was weak from when it was under PC, and the sudden change to lots of light was the final nail in the coffin.
Usually Clams take a while to actually die, unless it gets eaten or something, an LFS near me keeps them under far to little light, and they last about 2-4 months, but never longer than that.
If you were to get another
Crocea clam, I would keep it in the MH tank over the PC tank, I don't think the MH was too much light, I think the change was just too much.
Sorry about your loss,
Whiskey