hello,
I just wanted to say, that my 2 of the 3 cats i have; have had this crusted.. what i call dirty mouth, i took them to the vet for it and vet cleaned thier mouth area with poroxide and told me that it is like bacteria and dirt.. generally comes from them eating from plastic dishes. told me to use stainless steal dishes... they occasionally get it now and then but only since i used a plastic dish for their food, still use a steel one for water.. so thinking maybe they got it back because of dish.... hope this helps.