Again, I'd treat the yellow stuff as if it were part of an injury, with antibiotic ointment, or first aid cream. They can sometimes get injured, even if there is not a predator. Then proceed to treat the wing, as shown in the link I provided.
Looks like an injury. Did you look under the wing too? Put antibiotic ointment, or fist aid creme on the injury. After you check the wing over better, you might want to secure it to the duck's body with vet wrap for a few days, then recheck it, and rewrap it.
This is a good visual link for...