This is bumblefoot or a staph infection. You want to soak in Epsom salt water, apply neosporin, and wrap with vet wrap. Continue at least 2 times a day for a week or until the plug is easy to remove.
Once you remove the plug pack with neosporin and wrap. Change the wrap daily. It can take weeks to heal. Keep the duck dry until healed. Wear gloves as you can get staph from the infection.