It's hard to make a diagnosis with the dark picture and symptoms.
You say you feed beefheart and have had the discus for 3- 4 months. Do you have a gravel bottom in the tank? How often/much do you change the water?
A common problem we see here is dark discus, hiding in the corner, not eating, with stringy white feces. This is Hexamita (intestinal flagellates) caused by stress. Many times the discus water is not clean enough or the tank is too crowded. Beefheart quickly fouls the water if every speck is not vacumed out daily.
Your first line of defense is to increase the water changes and clean the tank. If you think it is hexamita you will need to use metronidazole sold under trade names like Hex-a-mit.
Metronidazole is most effective in water 92F (or a little warmer) I doe 400 - 500 mg per 10 gallons every day after a 50% water change for at least 3 days. Leave temperature up for a week or until they are eating good.
Carol