2yr old has had mucousy diarrhea on and off for 3 weeks, doctor ruled out parasites and food poisoning, what could it be?

Toddler diarrhea: It could be a slightly prolonged viral illness, could be C. difficile, could be a post-infectious digestive problem, could be lactose intolerance or other food allergy/sensitivity, or could be a condition known as toddler's diarrhea, where they get these types of stools for a few months for no apparent reason. If no pain or other symptoms, follow doctor's advice regarding further work-up.