Okay, so today, I paid Don a visit, and brought my camera along.
At first glance, I was a bit concerned about the clam.
You can see the "brownish" spots, on a few spots of the mantle.
However, then Don got me his step ladder, since I'd brought along my "look down box."
Unfortunately, his sailfin tang was continuously swimming over the clam, causing it to close. It did give us an opportunity to see that the clam is reacting fast, to perceived threats. The coloration is great!! Aside from the "spots" the clam appears to be totally healthy.
The pics are a bit blurry, but you can see the brown spots, along the mantle, that almost look like a burn.
However, the only thing in the tank that I could see, that would cause these types of burns, were a Frogspawn and a Torch coral, neither of which are anywhere near the clam.
I still think it could be a possible case of localized bleaching, but I'm not sure. Hopefully Barry will chime in soon, with his take on things.