Snail shell turning brown.. Why?

Hi, before going out I took a look at my snails.

A golden mystery apple (whatever that means bruh) snails shell is turning brown for some reason.

Why? Is it dangerous?

I don't see anything abnormal in the photos. The darkened spire is the body blocking light while the shell is translucent; the darker wiggly line coming out at the last whorl is the end of the digestive tract (so that's poop). If there is discoloration on the outside of the shell it'll just be algae (common for snail shells to turn brown/green/hairy from that), but I'm not seeing that in the pics.

Well I'll watch the snail for the time being. This is the one I don't see much of, it's off doing its thing lol.