eye problem on yellow tang

My yellow tang has a white film? over it's eye. I can't tell if the eye is gone or just covered with the the white stuff. It's swollen right over the eye area. He seems to be feeling fine, eating well, no more marks on him etc... I just noticed this a few hours ago. Can someone tell me what it might be? and tell me what to do about it? All help is appreciated. Thank you.