Being the clam is losing it's color I would bet it's losing zooanthellae meaning it's not getting enough light to grow this algae. Is this clam in the 60 gallon? What's the wattage of light on the 40 gallon?
Here are some options to try....
Add more lighting to which ever tank it is in. You want at least 5 watts per gallon. 7 to 9 watts would be most preferred for clams.
If the other tank has at least 5 watts per gallon, preferrably more, then acclimate it to the other tank.
The last option would to bring it closer to the top. Moving clams can cause damage so be very careful if you chose this option.
You can also target feed the clam a little extra food.
If the clam does start to die, it will turn pretty slimy. At that point do take it out of the tank. If it's meat is receeding, but not slimy, you can hospitalize the clam and attempt to save it. Good luck.
A dead or dying clam will smell aweful...like a monster size wet fart nasty aroma. The more rotten it is...the more dead it is, the stronger the smell.
...I don't know if this applies to all photosynthetic coral. I would imagine it does. I read an article about anemones and how they get their color and what role zooanthellae play in their biochemistry. Light dependent coral grow this algae in their tissues from the light they are exposed to. This algae, at least in anemones are utilized and transformed into sugar which fuels the animal's metabolic system. When anemones start to lose their browns and greens, this means they are losing this supply of algae and in turn losing the needed sugar to keep their metabolisms going. These animals can take a very long time to die. Sometimes up to a year. So it's very important that these animals get the proper lighting. Clams are high light dependent. Clams are generally raised in very shallow water and very close to the surface to achieve their light requirements.
A metal halide about 250 watts or more should help achieve these requirements.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.