Thanks for the picture. I really did not see that in the photo, but, yes it looks like you have some mild Bumblefoot going on there. If I were to see that, I would probably start soaking the feet for a few days in Epsom Salts to loosen the core, then once loose, with a pair of sterile tweezers...
The picture is a little blurry, so it's hard to make out the severity of the swelling, but from what I can tell there is no Bumblefoot, with your classic core and necrotic black-filled plug. Instead, it looks like pressure sores from being housed on hard/compacted grounds. Pressure sores do...
Good advice from Lydia. Could you try to get a video of the duck walking? To post a video, upload it to a video platform like youtube or Vimeo, and copy/paste the link here.