In most cases this dark deposit at the water line is caused by sun tan lotions, body oils, hair conditioner and so on. To help prevent it you can put a "scumbug" or "scumball", a special oil absorbing sponge, in your skimmer. I've found them at my local pool store. They are inexpensive and last for a few seasons. Just rinse them out in the sink when they get dirty. I've had considerable success in gently rubbing the scum line with a Magic Eraser. You do not want to use Soft Scrub or any soap products.