if you look at the pics, you will see the mantle is white on the flat part between the syphons ( holes ). the zooxanthellae algae that lives in the clams mantle has be expelled from that area, leaving it white. bleaching in this area is usually caused by light shock and can occur after changing older bulbs to stronger newer ones. to avoid this, you can slowly acclimate the clam to the new light by using screens or shortening the photoperiod.
it should get better ( quicker with acclimation ) but if not or it appears to be getting worse, please post another couple of pics as there are other reasons for this to occur.
HTH