It's odd that its not draining down the nostril. I mean its odd that there is the build up in the first place. But it's extra odd that its not draining. Something blocking the way? But if the duck is breathing without wheezing then there is no blockage I would imagine. So why not draining?
Some other part, perhaps? What's the anatomy of soft tissue in a duck's head? Could it be something to do with the esophagus or mouth instead?