Allen,
I can't tell from the picture, if your mantle is bleaching all over, or just in the central syphonal lobe. According to Daniel Knop in his book "Giant Clams" - a great read BTW, if the bleaching is uniform over the whole mantle, the cause could be low nitrogen, or lack of light. (As you said.) Do your nitrates stay at zero? - It would probably take months of zero nitrates for the clams to bleach due to lack of nitrogen. (Just a guess.) If both clams are affected, not just the one, maybe it is a nitogen deficiency. You did say that when you went without a skimmer, the mantle started to color up? Have you tested the nitrates? I am curious to know myself. The one derasa may just have more strored up nutrients and that is why it is not showing signs of bleaching. I do not know what your fish load is, but my first thought is that if you have hair algae, you probably have sufficient organic nitrogen compounds for your clams. But can't tell for sure without the testing.
It could be a light thing, i.e., an acute lack of light. I did not see a "before and after picture", so I cannot tell if the clam is browning up trying to create more symbiotic algae, or if that was it's original color. As Knop says, beware, sometimes the clams will be wide open trying to get more light, appearing to be fine, and then suddenly die. But, if the bleaching started uniformily, it could be an acute lack of light. (As opposed to just centralized bleaching between the syphons - where Knop hypothesizes in his book, too much light (or UV) could be the culprit.)
Have you moved the clam recently? In my experience, just a simple move can be the kiss of death for a clam. I put mine in the sand, mainly because they have the ability to move around and "get" to where they want to be.
I feel for you.

I have killed many a clam. Sometimes, they just cannot adapt and die. Good luck.