Looks like wet pox, but other things can cause that cheesy discharge in the mouth. Trichomoniasis looks like that (canker), it can affect chickens- though it is usually in wild bird and especially pigeons/doves. Do you have these around? Trich is easy to diagnose, but you need a vet or someone experienced with a microscope. Take an ill bird to a vet who knows chickens- or I would get your state poultry extension involved- you are in FL? Your University of Florida has a poultry extension- call them. If is Coryza, you can treat with several antibiotics, but recovered birds are usually carriers that will infect incoming birds- as well as a risk to all of your neighbors's birds. If coryza is confirmed (this is why you want to get them tested for a hard result, not just a guess)- most people will depopulate, sanitize, wait 30 or more days before getting more birds. Any idea where this came from? Have you brought any new birds in, in the last month?