The discus have cloudy eyes and slime coat problems. Even if the rays seem fine they are immune to whatever parasites they are harbouring and the discus are not.
I suggest adding salt (aquarium or pool salt is fine) at 1 tablespoon per 5 gallons and Kanaplex by Seachem according to directions asap. You've got some bacterial issue going on along with external parasitic irritant causing the slime coat reaction.
+1