Crocea clams like allot of light, and it is not really reccomended that they be kept under powercompacts, because unless you have allot of them the clam usually doesn't survive past a few months.
I would move him to the tank with the MH, toward the tank bottom at first, then in a week or so move him to the top of the rockwork. I think it will really appreiacate the move.
Also remember at 6 months PC's loose about 20-30% PAR, and it is reccomended to change the bulbs between 6-9, months. I have a light meter, and the amount of light really does drop off this fast.
HTH,
Whiskey