On our discus board, a lot of people have experienced discus slamming into the glass or a decoration in their tank and causing part of their body to turn black. It's like a giant bruise. Some also speculate that it's nerve damage in the affected area.
His mouth looks a bit swollen and strange in the picture you posted. Maybe he slammed into the lid or the glass? Does he have any tankmates? Maybe he got into a tussle?
lol thanks everyone... lol he was breathing heavly because we was "pooping" haha i fed him a pretty big piece of shrimp. but today he is doing good. other part of the face is still black as night lol. and i do agree it looks pretty cool