He might just have a slight eye irritation or, due to the color (green), it could be an infection. I took Tae into the vet back in February b/c her eyes became goobery and one morning the fur above and below one eye was a bit goobery. The vet gave me some ointment to put in Tae's eyes. I've read about a lot of Boxers having similar issues and the vet usually gives an eye drop (my vet's the only one who gave an ointment) to clear things up.