It's fine, they expel zooxanthellae when in excess. Also, waste as well. Can you take a pic of the whole clam with whites on, no blue light pics. Looks like a nice maxima.
Maximas usually settle in rock or coral heads and are not found on the sand. If it's 6", then most likely wild caught. These guys need a lot of full spectrum light, my guess is it's in pretty low lighting for what it requires and should be moved up the rock work. I would shoot for 300+ micro moles or more. There should always be new white growth at the upper margins of the shell, just under the mantle. If you haven't done so already, pick up James Fatherree's new book on clams. You can buy it here.