My kitty had this. You can try a little hydrogen peroxide. You want to dry out the areas. Be careful not to break open the pustules or it could spread. Just have someone hold the kitty and pat a peroxide soaked cotton ball or q-tip onto affected areas. It may sting a bit for the kitty so be prepared and work fast. The vet also carries pledgets specifically for this that you can try. If it is getting bad quickly and is bleeding, you may need a vet visit for oral antibiotics.
Side note: some cats get this from eating out of plastic dishes. Switch to crock or glass dishes to see if this helps.
Caretaker of 2 dobermans,, 1 schnauzer, 5 cats, 3 goats, 1 parrot, 1 bearded dragon lizard, 11 chinchillas.
Rest in peace, Cricket, Casey, Bella, Yank, Chloe and Bones.