Any experience with localized bleaching on clam mantle?

[Delphinus]

Hmmmm. Not impossible I suppose but seems unlikely? It sits in a spot where there's no saltcreep above it.

However ...

It does sit underneath the nori feeding station for my tang. Not sure if that's significant or not.

I'll try to take a picture, thing is as open as ever, just happens to have a spot about finger-sized that's pure white on the mantle.