Excellent link!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 wrapping the wing. Remember not to get it too tight.
https://www.gardenbetty.com/chicken-first-aid-how-to-wrap-a-sprained-wing/