The mantle of the clam is pulling away from the shell itself. This is not a good sign at all. More commonly this will be a chemistry problem or parasite (pyramidellid possibley). As long as you are acclimating the tank properly and slowly enough to the MH
light, this will not be your issue. From the pic I saw, you are going about it correctly.
Please post the alkalinity, calcium and magnesium levels. All three
of these are very important so if you do not have the kit for them, get them tested by the LFS
and post the results.