It could b a parasite. But when I kept discus, stress alone could cause the white stringy poo. I was told that it was the intestinal lining.
Is the fish still eating? If it is, might not b parasite
If it is a parasite, move it to another tank and treat. Discus like to eat food off the bottom of tank and that's where the poo is with the parasite in it. ( try to keep the other fish from eating parasites)
Hope this helps a little
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