Eyes/Non-crusty White mark on eye?

I can't really tell from the picture, but see https://www.guinealynx.info/eyes.html and see if any of those pictures give you any help.

It looks like heterotopic bone formation to me (the, I guess, older name for it is osseous christoma/osseous metaplasia as referred to on Guinea Lynx). It's essentially when bone tissue forms where bone tissue normally doesn't form. There's a great paper on ocular disease in guinea pigs here, (broken link removed) which has some information on the condition, as does Guinea Lynx. I have a few pigs with heterotopic bone formation in their eye(s). It can lead to poor eyesight or blindness as they get older, I find. But in some piggies it doesn't seem to affect them at all.

I actually don't, @spy9doc, I just magically found that paper through luck and the Google machine. It's an interesting and informative read, certainly.

Thank you so much! That does look quite similar to Marshmallow's problem. I'm going to be taking him for a vet visit with our exotics vet to get it looked at. Thank you again.