A clear or opaque discharge from the dog's eye is fairly normal. It can be caused by allergies, something in the eye, sleep, etc., -- just normal stuff, like the crud we clean out of our own eyes.
Yellow and/or greenish discharges usually mean some sort of eye infection. Again, in puppies this is a fairly normal occurrence, such as with skin kids and ear aches. They get them and they are easily treated. No big threat.
Call your vet and see if he wants to start some "drops" or antibiotics since it is a yellow discharge. Could be the start of some infection.
In the meantime, simple saline solution can be helpful in rinsing our the eye and removing contaminants, allergens, etc. Might try that before a visit to the vet to see if it clears up.