It can also be a mucas that comes from the eyes. When I first got my girl she would have a glob of greeny-gunk in the corners of her eyes each morning and throughout the day there would be traces as well (she would just wipe it away by rubbing her face against her leg).
After a couple months with me on a high quality food she doesn't have anything and her eyes are clear. The eye gunk is one of the body's ways of getting rid of excess toxins or indicates a slight allergy (to food or environment). In more serious situations the eye-mucas is a sign of a bacterial infection and needs to be seen by a vet. When I got my girl I had the vet take a look and he said it was likely allergies but could be diet related, after the change in diet and her being mucas free -- we decided it was most likely from the diet but who knows, could be an environmental allergy that comes back with the warmer weather as I only got her in Sept. Time will tell!