Hi
You will have likely not noticed because is hasn't been there before or has developed so gradually that you will have easily missed it.
It is just the lower eyelid falling away and exposing the conjunctiva; something that is harmless and not all that uncommon, especially in older guinea pigs. There is no treatment. It usually called fatty eye or pea eye.
If in doubt, please see a vet at your convenience during regular opening hours for a potential conjunctivitis. The conjunctiva doesn't look irritated or swollen to me, though.