Well at least you've identified the problem. No sense in beating yourself up over it tho. I'm still seeing an awful lot of mud on the leg in the pic. If that was my duck, I think I'd bring it inside and absolutely continue the Epsom Salt Soaks 4X per day as before, that will aid and assist...
Oh. Start with the Epsom Salt Soaks! Is that mud on the leg? I think you want to soak that off right away. No soap. I would not pick at that stuff either, just soaks. 4x per day, exactly like Amiga told you. Start there.
Edited for spelling