Dogs vomiting white mucus may be caused by stomatitis, reflux gastritis, pharyngitis, high fever, poisoning, and parasitic infection. Dogs vomiting white mucus may be indigestion or illness. If the dog eats hard food or eats too much at one time, it will cause indigestion and vomiting. Gastritis or parasite infection can also cause your dog to vomit. Take your dog to the hospital as soon as possible.