Mucous can be be caused by malabsorption, fecal incontinence, inflammatory bowel disease, or colitis amongst other things.
At this time, some kind of lower endoscopy - either a flexible sigmoidoscopy or colonoscopy - would be the most reasonable and comprehensive options to evaluate for the conditions mentioned above. You can also consider tests to evaluate the stool for infection.
This can be discussed with your personal physician or in conjunction with a GI evaluation.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
There are several possibilites, including proctitis (inflammation or infection of the rectum), fistula (a tract between the anus and the skin, usually resulting from an abscess), internal hemorrhoids. Some tumors cause mucus discharge, especially benign (non-cancerous) ones called polyps. The only way to find out for sure is to see your doctor. It's by far most likely to be nothing very serious, but there's no way to be sure without an exam.