Canker is contagious to the other members of your flock. If you can seek vet care, they can confirm that it is canker, a protozoan disease. A necropsy by your state vet if you cull can confirm the disease. Other diseases can cause the yellow plaques inside the beak , and they include wet fowl pox, ILT, and some bacterial diseases. But the description of them having a bad odor and spreading or growing, sounds like canker, which I have never dealt with. It may have already exposed to other members of the flock, since it is spread by pigeons usually via water sources. I would scrub out and bleach your feeders and waterers.
Metronidazole (Fish Zole, Flagyl) and ronitazole are medicines to treat it. Fish Zole is available online, and the dosage is usually 259 mg daily for 5 days. Sources say that removing plaques can cause bleeding, so be cautious removing them. The plaques can grow so large in a short time to block the airway and make it hard to eat.
Acidified copper sulfate may be used 3 days a month to help control the spread of the disease. Dosage is 1/4 tsp per gallon of water.