Just captured this. I have seen it the last few days actually. It appears the platy is cleaning the discus. Not just this one but a few in the tank. Anyone see this before? Seems the discus position themselves for a service much like Tangs and cleaner shrimp.
Not with discus (goldfish), but yes, livebearers are picky little things. Goldfish don't appreciate it nearly as much as your discus do though. Cool video.