I'm sorry to say, but it may be dying. I know goldfish do that when they are dying. I understand that your fish isn't a goldfish, but that doesn't mean it can't have the same effect. Or, it's colors are changing. When it happened to my goldfish, my friend told me it was dying because that's what hers did. But instead, my goldfish lived and it has black stripes and some white spots. Some fish change color when they get older.