Eye boogers. Your baby may have normal mucus buildup at night, or may have a blocked tear drainage duct. Call you local pediatric ophthalmologist to have this checked soon.
Blocked tear duct. The tear duct is the drain for the eye, located in the corner near the nose. In babies it often gets plugged up, causing the material which would normally drain through the duct to pile up in the eye. Usually gently cleaning the eye with a warm cloth can help. If it continues, see your baby's pediatrician.