I have a 20 yr old hen, and since las yr I noticed her whole mouth , tongue , is Black ?, thought It strange when first noticed it , but it doesn't,t seem bother her, it not a slime , I just put it down to old age , she doesn't go out loft much , but if it had been slime You hav to watch drinkers ( other birds) I not a clue cause it , but doesn't bother her so Doesn't bother me , Just old timer seeing out end of her days , hope u find out Atb