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This is milky cleaning fluid, which is normally re-absorbed but can on occasion spill out. Unless you see it all the time (which could indicate a blocked tear duct), you need not worry.
Watery tear fluid (drying to a transparent crust) is always a sign that something has got into the eye or that there is a discomfort or injury in there, especially if the eye is drawn in. if your piggy is blinking and watering a lot, please see a vet promptly as your piggy could have a potential eye injury hidden underneath. A little one off crust is generally just from a fleck of dust being washed out and nothing to be concerned.
Greyish gunk is the sign of an ulcerating (i.e. infected) eye injury.
Greenish/yellowish or orangey gunk in eyes and nose are typical for an advanced un- or undertreated respiratory infection whereas transparent crusts around the nose are dried watery discharge from sneezing (the normal reaction to clear the nose).