It seems like shell erosion to me. Shell erosion happens when their is a defiecency of calcium in the aquarium, not enough calcium in the aquarium for the nerite.
Calcium helps keep the shells of snails looking nice, hard and healthy. When there is not enough calcium the shell starts to kind of like peel of.
You can try feeding your nerite foods rich in calcium and see if that helps improve the shells, they are picky eaters so they might not accept the food.
You can put your nerite in a cup, crush the food rich in calcium and spread it all over the bottom of the cup that your nerite will go into and that might help him eat the food. You can do this during your water changes.
How old is your nerite?
Usually the older they get the more their shell erodes naturally over time. With my Nerite that is around 3 years old, his shell was starting to erode also and I have tried feeding him calcium rich foods but he never accepted them. And I knew my water had enough calcium since I also have cherry shrimp in the aquarium and they also need calcium to molt and I never had a problem with them molting at all. Also my GH in my aquarium is around 8 dgh so it has a decent amount of calcium present. So I just left him and he still is alive and moves around the tank.
I would test your gh to see the calcium levels in your aquarium.
It is also possibility that it could be an injury also, I believe that the shell will heal naturally, you will just have to stay on top of your water changes and make sure that your water stays clean. And his shell well regrow naturally.
My Nerite
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