Tooth decay is NEVER caused by mange. And it cannot be fixed by “just” brushing (if that is what you see) VERSUS (for example) stained teeth from a bout of distemper, heavy tartar build-up, or bad wear from chewing on fencing, etc. Distemper can also cause a purulent eye discharge and pustular dermatitis. There is no way for any of us to *know* if the dog has that, or mange (or which TYPE thereof) or multiple issues (like food allergies – which could also cause goopy eyes) or an eye infection etc., etc. etc. Don’t WAIT => see the vet, ASAP.