That happened to me also, about four years after my colostomy I started having a discharge, it took me for ever to find out why it is happening. When we had the surgery that left a stump of a colon a few inches. We have mucus membranes in our colon to keep it moist and help the bowel movement pass easily. After the surgery we no long have anything passing, but the mucus membranes are still there and they become active because they have nothing to pass they pass the mucus discharge. I have never found anything to stop it.
One person a year or so ago told me that it is because we have no more short chain fatty acids going through or colon stump. He had actually tried fecal transplant to solve his problems. I actually tried that for a month or so and my doctor told me it was not a good thing to do so I stooped. My doctor says to just use an enema every morning to clean it out. It wont stop it but it keeps it clean. Good luck. I hate mine because it is having drainage all the time. I have found no solution.