He's and indoor cat and he does close his eyes a lot, I hope its not scratched I'll ask my vet definitelyHi klaus_and_remy and welcome to TCS.
Does your cat go out? Is he allowed outside?
Have you seen him pawing at his eye, or keeping it squint or else?
It seems he had his eye scratched by something. The thin layer of tissue that covers his eyes could have been torn by a shallow scratch.
It really looks like what happened to a cat I had many years ago.
Could you send the photo to your vet? They could tell you if it's what I think. And also what to do.
Fingers crossed for your kitty!