Hello,
That behavior is "sometimes" used by discus when they they have an external irritant.. like a parasite problem. Ive seen it a few times. Sometimes the other discus will be attracted to the fish and appear to pick on its sides a little.I don't know if thats an evolutionary fall back to dealing with external parasites but its what I think it is.
In your case it fits... new fish, flashing, health issues. Im a bit concerned as your tank is over stocked and thats never a good thing if you may have a parasite issue starting..
The Api General cure is just metronidazole (200mgs) and prazi (75mg) I believe. Though its called "General Cure" its probably limited in how effective it is. Metronidazole is dosed at doses of 250-500mgs for up to 10 days usually with discus for ex.
I think I would try Salt dip on the fish in question .And then run a course of seachems paraguard in your tank..
or if you have it, formalin with malachite green..Unfortunately the loach and otto cat may not survive the treatment.
If this is a parasite issue starting it could be very hard to get rid of because of the substrate so best to knock it out know before it really gets established.
I could be off the mark here entirely but thats my best guess. I subscribe to the school of" better safe than sorry".
hth,
Al