Like NorwegianDragon said – if it is kind of a cheesy substance – you need to see a vet, it could be mouth rot.
If it is just the inside of the mouth itself that is yellow it doesn’t *have* to something wrong. But it *can* be something wrong with the liver – then it has to be taken cared of. I had my dragons checked because of their yellow mouths just before Christmas – and it was nothing.
But like I said, it could be something seriously wrong as well… Since your dragon also stopped eating (which isn’t quite normal for a dragon at that age, really...) you should take your dragon to the vet and find out if/what is wrong with your dragon – then you know how to treat him. It is impossible to say what it might be, without have seen the dragon.
Good luck!
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