150W MH seems like enough light to me, if the clam is close enough. 70 cm, that's about 2.7 feet, right? That's one deep tank! That might be the problem. Perhaps you could try to move the clam closer to the light. Also, you might want to consider supplementing with VHO or PC. 150W is the weakest of the MH light setups, and might not be enough light if your tank water is yellowed (might want to run or replace carbon to clear it up if that's the case) or if the clam is far away from the light.
Looking at the picture, it's hard to tell what's going on there. I would move the clam a little closer to the light and wait and see.
Squamosas are not as light-crazy as maximas or croceas. I think a little closer might do the trick. I would not move it too close, or too much closer all at once. Do it gradually, a few inches per week, until you're about halfway up the tank, and then wait to see if the clam colors up better. I don't know if that will work, but that's wwhat I would try first.
A side note: Again, I don't know as much about squamosas as I do about the other clam species, but I know that croceas and maximas have less "center" and more "side" to their mantles than the other clam species. Sometimes this narrrow center strip is less colorful and can look whitish. This does not usually indicate that the clam is bleaching or is sick. I do not know whether this applies to squamosas or not.
I think Knop also mentions that sometimes in a tank that's very clean (no nitrogen products and exhausted of trace elements), a clam will sometimes bleach out despite good lighting. I don't know how relevant that is, but it might be something to think about.
Two questions:
How large is the clam? I've read that larger clams need disporportionally more light than the smaller ones...
Also, is (was) your squamosa brownish and dull, or was it very colorful? If the latter, it's even more likely that it needs to be closer to the light. According to many writers, the more colorful a clam is, the closer it was to the surface in the wild or on the clam farm, and the closer it needs to be in one's tank.
HTH
