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.