Is Clear Stuff Coming From A Dog's Mouth Bad? What Does It Mean?

This clear stuff that you are mentioning can be saliva. This condition is called drooling. Dogs can have drooling to the stimulus of food, heat and excitement. Sudden and excessive drooling in dogs is not good. This problem can be due to

1. Gum & dental diseases
2. Tongue injury
3. Stomach disorders
4. Mouth ulcers
5. Tumors in mouth
6. Some metabolic disorder
7. Rabies

So, take your dog to vet if he is drooling excessively and suddenly.