Sounds like the shell has started pitting. Can occur is the snail is not getting enough calium in it food intake. How high is high for you PH. If the snail is new may have had the damage done by living in its last tank.
Ph sounds fine, pitting is a process it wont occur over night it may have had tiny marks when you got it and there getting bigger as it not getting it calium. Can you post a pic as im kind of shooting in the dark without seeing it?
any chance he got damaged in transit, or has fallen in the tank? looks more like healing damage to me...
Do you have any other snails/bottom feeders in the tank?
If you can get some cuttle fish, Dried that People who have birds use. It will enhance the shell and make him stronger.
yeah thats the stuff or you can try feeding some high calcium feed (turtle pellets?).
There is a chance (slim) that the damage might have bee cause by the fish, but I think, it might be rasping by the nerite (did he perhaps have a little algae growing on his shell?).
Hope he heals up!