It's just where the outer surface of the shell has eroded. All my snails have areas like that on their shells, with the 8 1/2 year old zebra nerite having a large amount of eroded shell. Judging by my nerites, it doesn't seem to affect them, just makes them look not quite perfect.
Yes. I bought a zebra nerite and one of those red ones with black markings that you see called red onion or tyre track in July 2011.
This is a photo I took of the zebra nerite in October 2018, as you can see it's badly eroded but it's still crawling round the tank 15 months later. The white band is quite hard it's just where the surface layer has come off.
The second photo is the other snail I bought in 2011, the photo was taken in April 2016.
I wish the LFS sold more colorful ones like your, I am happy with them for what they do. I miss the color the Golden Mystery Snails add to the tank but not the damage they did to my plants. I now have several ramshorn snails in my shrimp tank they are hitch hikers. They also are a shade of brown.
Where I am I have a Petco that seems to only sell apple and mystery snails and the LFS that sells apple/mystery and brown nerites, I have 7 in my 55 gallon and 2 in my 30 gallon. So I guess I will have to buy online to get something different.
Unfortunately there are none close by, we use to have 3 LFS but no more. The next closest store is a Petsmart and I did not like the look of their fish, they seemed stressed and too many sick fish in their tanks. They did have a nice selection of tanks and supplies. I would have to travel over an hour away to the closest LFS.