I think, but not 100% sure, that your clam is bleaching a little. Did you change your light? Did you move the clam to a different place in your aquarium? Check the bulb make sure that it is not cracked. Was there a heat spike recently? How are the rest of the corals and clams in your tank? Did you change water the last few days? If you did, did you change salt brand, or have a new container of salt of the same brand?
I find that stress to your clam can cause minor bleach to total bleaching. You just need to determine what it was and try not to do it again.