Where are you located? If you're in a dry area and tend to lose water from evaporation, when you add more water to top off the pool, you're adding more calcium into the water. The calcium does not evaporate with water and will continue to build up over time. Are you superchlorinating your pool on a periodic basis? If so, what are you shocking with? If it is a calcium hypochlorite product, that will contribute to the increase in calcium hardness also.

The salt system should be designed with a self cleaning feature built in, but if it shows signs of scale, your water chemistry balance is out of balance. As mentioned, lowering your pH will help. However, if you have excessive scale formation on your tiles, you will need to ensure you maintain a lower pH (some may call it an acid bath) for a few days to dissolve the calcium. There are some calcium remover solutions that require some elbow grease to get the calcium off. In some extreme cases, I've heard of needing a chisel to get it off, but is not usually recommended. You can damage your tile!