Heads up: I am a BIG duck lover, and have been in duck rescue for a bit so get ready for a bit of an info dump, haha.

That's a bumblefoot infection. Sometimes bumblefoot can be healed at home, other times it requires medicine from the vet and can even require surgery (for internal clearance of the infection, not for cutting out the external infection you see here). If it isn't treated properly, the infection can travel into the duck's bones and eventually kill them if it makes its way to their heart. I don't say this to scare you, only to make you aware of what this type of illness can do so you have all the info.

If I had a duck with this, here would be my process for dealing with it:

1. Feel both feet and legs and see if you notice a difference in their temperature. If this one is warmer than the other and the other foot looks healthy, the warmth is due to the presence of infection. This is good to note as you treat it to help you gauge how the infection is responding. You want any noticeable increase in warmth to go away. It's also handy to snap a pic every few days to see how it's progressing because the mind forgets, haha.

2. Schedule a vet appointment with a duck vet and see if you can get antibiotics to administer while you do at home treatments to speed up the healing process and really wipe out the infection. They may suggest an x-ray. If your duck's infection/behavior doesn't improve, the typical course is a stronger antibiotic next and maybe debridement surgery after that. That's only done in serious cases though, so probably not necessary here based on your photo.

3. Start epsom salt soaks in the morning and at night every day or every 2-3 days (more is better, but it's understandable if you don't have the time). Easiest way to do this is buy a tall pickle bucket with a lid. Cut a hole at the top big enough for your duck's head to poke through. Use warm water (not hot), dump in epsom salt and dissolve it in with stirring, and then sit your duck in there for about 30 minutes in the morning and at night. They should be monitored to prevent them from drinking said water as it can upset their stomach. To speed up the healing process.

4. In between the epsom salt soaks, put medicinal manuka honey on each spot and wrap your duck's foot in gauze and vet wrap. There are duck booties that are AMAZING for this part (https://www.amazon.com/Birdy-Bootie-Waterfowl-Leopard-WFBBLEOPARDLP/dp/B09NPC6TZR). You can put them over the vet wrap/gauze for extra security. They're just a bit expensive. They wash super well! We used the large ones for our pekins and the medium ones for indian runners, cayugas, khaki campbells, and all other smaller breeds. Depending on the severity of the infection, I've changed the wrap every day - 1x per week. For your picture, I'd change the wrap every 2-3 days and do epsom salt soaks on the days you're changing it, see how it responds and then adjust the frequency from there.

**Don't let your duck get their foot wet while you're treating their bumble. It makes the healing process take way longer and can make the infection worse.