clam help

[cyberdude4747]

I have a tridacna maxima and it's mantle is starting to develop light brownish spots on it does anybody know what this could be caused from it seems to be doing fine.I think that I have sufficient lighting for it I have a 250 watt hamilton 12k and 20k radium 250w and 100w flour. actinics
water quality is perfect clam is about 12 inches from the surface.
Please help