is this normal on my snail?

hiiii sooo this is Crao, our (my bf and me) snail we saved him and another one almost a year ago, the other one unfortunately didn't make it since his shell was way too broken to be saved even with all our efforts.

we we're just wondering if this white spot on the back of his shell is normal, it appeared in the couple last weeks, my boyfriend suppose that it is either a lack of calcium or an excess of calcium, he gives it some regularly.

we saw that other "outdoor" snails had some too so is this normal? we are parents chickens so please reassure us <3

(ps we have clean hands to not contaminate either crao or his food)

https://preview.redd.it/dtaz1ukaa0fe1.jpg?width=1393&format=pjpg&auto=webp&s=cef863b11925aba546d7bf71775e674fa28789dc

https://preview.redd.it/2gpqa00ca0fe1.jpg?width=3024&format=pjpg&auto=webp&s=e43f09f38cb16df301885049ccd8ca61b816f1f4

https://preview.redd.it/r7hd9x0ca0fe1.jpg?width=1532&format=pjpg&auto=webp&s=df375587807bd0a37e6d5fc3907e1b8df69f263d

https://preview.redd.it/fv60c81ca0fe1.jpg?width=1167&format=pjpg&auto=webp&s=da4835f368932c72de6c090f01192a284b8d9fb3

https://preview.redd.it/f669gw0ca0fe1.jpg?width=1036&format=pjpg&auto=webp&s=6b0f59a2d5ffdc9123dd0f6d2752f38c46919aea

https://preview.redd.it/3upb771ca0fe1.jpg?width=1601&format=pjpg&auto=webp&s=0d3debf2122d5ed7721fb8fa4a713293cfc392d1

https://preview.redd.it/6iuazv0ca0fe1.jpg?width=1338&format=pjpg&auto=webp&s=a56e8c7133124829a4d3974d286aaac5ed67bdbf