Sound like Hexamita, a protozoan parasite that many say is always present to some degree, but needs to be continually managed with Discus. Selective bred Discuses are very susceptible to it, part of the tradeofffor generations of breeding for color and beauty.
I treat mine for it with Metronidazole. I prefer the SeaChem product about $9 US. Also use the SeaChem Focus about $7 US. Mix it per package instructions with about two cubes of blood worms and feed it to them, if they are still eating. If they have stopped eating then you can treat the water but I recommend a smaller quarantine tank so the medication goes father.
When treating the food using a little less is more effective than using a little more. Not really an over dose concern, just that the fish can taste it and so less bad flavor ends up being more that they eat. The Focusproduct is a binding agent that helps it stick to the food. Works well for other meds too.
I treat mine preventatively every month or so.
I have also heard that raising the tank temp to minimum of 95Ffor a few days is an effective treatment. I havenot tried this myself because I cannot find a tank heater that goes that high,but I trust the source and would try it if I could. Just be sure to oxygenate and be aware of otherspecies that can’t handle that temp.