Hi
That is a protracted tear gland a sign that the conjunctiva is somewhat swollen.
Watery fluid/transparent crusts and a retracted eye (the latter is always a sign of pain) are usually signs of an injury like an eye poke or something scratchy stuck under the lid. Since eye injuries can deteriorate very quickly and the extent will only show up with a special dye under UV light, please always have a piggy seen within 24 hours if possible; if there is bleeding in the eye, then this counts as an emergency.
Eye treatment and medication is the same for guinea pigs as for other species, so with any eye problems please see the vet within your reach that can see your piggy soonest over seeing an exotic vet. Speed really counts for a much quicker and smoother recovery.
PS: Normal eye cleaning fluid is milky white since guinea pigs don't clean their eyes by blinking; gunk from an ulcerating eye infection is a light blueish/grey and thick mucus from an upper respiratory tract infection is green/yellow/orange. Watery fluid is always a sign of an irritation in the eye.
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All the best