Wish I had seen this earlier. I'm with you it's not a result of the WC
. Your fish is stressed for some reason and appears to be losing its slime coat. Discus require LOTS of water changes and excellent water quality. Some basics before going any further.
What size tank ?
Other tank mates ?
Other water parameters ?
Did you QT this before adding ? (From what you stated it doesn't sound like it)
Are you treating the water ?
Parameters if water right from the tap ?
Edit : Discus do best in 10-15 gallons per fish and is recommended for a min of 75 gal