There could be several issues. One, nothing is wrong as swans will hold at least one leg/foot on its back. There are so many capillaries and other blood vessels in the foot that many researchers believe that this is a way for the swans to cool. The capillaries may help dissipate body heat similar to why a dog pants.

The brown crusty bumps could be a staph infection which can cause a large bump on the bottom of the foot called "bumblefoot".

The brown crusty sores could be caused by a fungus. If you think that the swan may have a sore foot and possibly impaired by pain or infection, you might want to contact the local RSPCA or Wendy Hermon, Swan Lifeline, Eton to take a look at the swan to ensure that he is okay. If they think it may be a concern, they will capture the bird, treat it and then return it back to its mate. The Regal Swan