My Runner had almost the same thing I took her to the vet because I wasn’t sure if it was bumble foot since it didn’t look like any bumble foot I had seen. She was on the antibiotic for a week along with mextacam for pain an inflammation. I then took a scalpel and open it up from the top and mashed till the core pus popped out. I then sprayed Veterycin on it wrapped in in vet wrap and left it for a couple days thinking I might have to get more pus out but it had already started to heal and swelling was already going down. It took about a week for the swelling to go down all the way. And pus in ducks isn’t runny it’s thick and cheesy looking. Any chance you can take your duck to the vet?