Scale deposits can form along the waterline of your swimming pool. This is usually caused by either high pH, high alkalinity, or a high calcium concentration. Constant temperature changes along with rapid evaporation will cause deposits to settle along the side your pool wall. If the deposits are not removed soon, it can cause permanent damage to the pool surface.
The first step it to stop all swimming pool use until a water test can be performed. If the test reveals that the levels are within range, you can use a stain and scale remover to remove the deposits from the water line. If the pH, alkalinity or calcium levels are too high, they will need to be lowered before using a stain and scale cleaner.
I recommend starting by lowering the calcium hardness first. Calcium can only be lowered by partially draining and refilling the pool. Your swimming pool needs a calcium level of 200-400 ppm. A calcium level higher than 400 can lead to scale of the pool surface and equipment and can also cause cloudy water.
Once the pool has been drained, re-filled and the calcium is back within range, you can adjust the alkalinity. You will want to re-test before doing so because the alkalinity should be lower than before. To lower alkalinity, you can use either dry acid or muriatic acid. The amount needed of either chemical will vary depending on the pool size (gallons) and the alkalinity reading. If the alkalinity needs to be increased, you can follow the instructions on:
How To Maintain A Swimming Pool Part 3 (Adjusting Chemicals)