To me, it looks to be just cuts and abrasions, likely caused by the bark that you're using in their coop. If people checked their duck's footpads more often, they *might* notice it looks like this right before infection. The cuts act as a gateway for the bacteria to enter, so it is something that should be taken care of before an actual infection starts.
I would first start by correcting whatever husbandry issue you think caused this, it sounds like you think that it was the bark bedding in the coop. Well, change it. After that, you could soak the feet in Epsom salt water to loosen the scaps and flaking, then slather an antibiotic ointment occasionally.