I don't do discus but clear, stringy feces in either discus or other cichlids is usually hexamita, a protozoal infection, many fish have it but never have problems from it until stressed, then it makes the fish ill and can kill it, if one fish has it odds are good all of them do, It is best to treat both the tank and the fish, The medicine for this is Metronidazole, powder or liquid for the tank, and treated food for the fish, follow the instructions on the package as concentrations vary depending on brand. Most call for daily feedings once per day for between 5 and 10 days. If it just goes away on it's own it hasn't really gone away, it has just gotten better for a while. The good news is that even heavily infected fish usually recover if treated. It also helps to thoroughly clean your gravel towards the end of treatment.