It looks like a calcium deficit, I'd get some cuttle bones and chuck them in the tank.
Hello; This makes some sense and is similar to what I am thinking. If a needed mineral is lacking such a result could happen.
I was thinking more along the lines of a food source having an influence on the shell color. Not so much a deficit of something as the food source available adds something which might be incorporated into a shell.
There has been a spoiler in my tanks in the last 50+ years. I have had snails nearly the whole time. Plenty of calcium in my water and 99% of the snails have a common shell color. However a few have grown out with white shells. May be a genetic difference or some other sort issue.
