Unexplained behaviour with discus fish...?

Pecking is not good. That the fish is letting another fish peck at it is even worse. That is not a healthy fish-- it is physically unaware, and probably quite close to death. You'd best get that sick fish into quarantine ASAP, and start dosing the tank with potium permanganate to reduce the microbial viability of the water, then attempt medicating the food with metronidazole and praziquantel. I'd recommend doing the same for the fish in the main tank, as there's a good chance whatever you're seeing has already spread. How big is the tank, and how many fish are in it? If you couldn't tell that something is wrong with this fish, I'm guessing you're a poor judge of stocking, as well.