Any experience with localized bleaching on clam mantle?

[Delphinus]

Noticed a patch of white on my T. squamosa's mantle tonight. Clam is not gaping, in fact looks otherwise basically healthy, and this sort of came out of nowhere.

I read up on Knop's clam book and he says (going to try to paraphrase a bit here), it can happen, it happens quickly, and so long as it doesn't spread and turn into "generalized bleaching" (ie. the whole mantle), the clam can recover in a few weeks. Generalized bleaching, on the other hand, he says is basically fatal.

Soooo .. um. This happen to anyone else?

Knop talks about causes of localized bleaching generally resulting from changed in lighting intensity ... I haven't changed lighting recently so not sure what to think about that. My actinics are about a year old, I suppose it wouldn't hurt to replace those.