That could possibly be canker (trichomonas,) a contagious protozoan infection that is carried by pigeons. It is said to smell bad and cause yellow to white gunk inside the beak, and can be treated with drugs such as metronidazole and ronitazole. Fish Zole sometimes is abailable for tropical fish ans may be useful. If you a veterinarian or access to any of those meds, they might be helpful. Acidified copper sulfate can be used in the water 3 days a month as a preventative so the disease does not spread to other poultry. Here is some reading about canker:
https://www.chickenvet.co.uk/news/post/a-review-of-oral-lesions-in-backyard-chickens
Wet fowl pox can also leave similar patches inside the beak and throat, so if you have seen fowl pox scabs on any chickens that could also be the problem.