Mucus is an indication of irritation in the bowel. This may be a transient problem from something your dog ate that caused bowel irritation, or the beginning of something more serious. If the dog is feeling well otherwise and still has a good appetite; try feeding a soft bland diet for three days.
This would include baby food, boiled hamburger or chicken mixed with cooked rice or potatoes. If it is a transient problem the mucus will probably go away, and you may then transition back to regular food. We suggest a diet with a unique protein and carbohydrate source as a matter of course.
Our food has a base of bison and brown rice. If the dog is ill and has a poor appetite you should immediately pursue other diagnosis. The rule outs would include food allergies, inflammatory bowel disease, and bowel tumors. Laboratory work and radiographs would be required.