Some people who have an ileostomy still have part or all of their rectum. The rectum is living tissue, and will continue to produce mucus. This mucus may leak out of the rectum at times and a pad can help to 'catch' the leak. It may also help to periodically sit on the toilet to try to expel the mucus in the future. However, if the amount of mucus is excessive, particularly bothersome, has a foul odor, or is green or yellow or red in color, it needs to be evaluated. Push the doc to follow-up.
Your dad should not be having any bleeding or discharge, especially since they saw no obvious signs of lesions or hemorrhoids. It sounds to me like his bleeding is a result of the surgery, and should be investigated further. I've had five people in my family who had their colons removed and never anything coming from the rectum at this point. It's closed off and nothing should be getting through.