My first thought was that it looks a bit like a boil about to erupt. I don't have as much familiarity with veterinary medicine as human though.
I'd probably be inclined to try and clean the area with warm soap and water first. Depending on if the white head disappears/pops/remains I would probably try and get into the vet. Boils are a type of abscess, usually can be treated with lancing and/or antibiotics.