Hey, my beardie has a few symptoms - apathic, doesnt eat much,sunken eyelid, uninterested and almost not responsive. Part of his urea has yellow colour and is hard when you touch it. In his mouth there is a few brown spots ( photo and video).
Just looks like some plaque or food debris to me. It doesn't look infected or damaged the way it would if there were an infection. You might be able to clean that up by offering some hard shelled food or use a q-tip to gently rub the area clear.
Has he been losing weight? I would track that. If his weight is stable he may just be going through a phase in which he isn't as hungry. You can offer some extra hydration by dripping water on his nose so he can lick it up if he wants.