Any experience with localized bleaching on clam mantle?

[christyf5]

I don't know about "localized" bleaching. But my clam used be a uniform dark color with mottled blue striping. In the past 6 months or so, the outer length of the mantle has turned a greyish color with black spots, much like a "gold" maxima. The inner part of the mantle is still the dark with blue. Its actually quite pretty but kind of weird when I look at old photos of it.