face

okay... so was just out of town for a week, and came back home to one of my gymnogeos having a wierd face thing going on... half of it has turned black... straight down the vertical dividing line down his "nose", etc, then around back around his gill cover... it does not extend down to the lower...

I just noticed today that one side of my female Aulonocara sp. "Chitande Type North" face has turned black? why is this happening? Does anyone know what causes it & how do you get rid of it? Could it be a parasite parasite that is doing this? What should I do to treat this condition?