What do you use to clean the salt line off pool tiles in a saltwater pool?

If it is "salt" it would dissolve with water. More often it is alkali or calcium that has deposited at the water line. If you catch it when it is just a "film" it will usually come off with an acid product. I prefer to use Bio Dex 300 but there are others on the market. Be extremely careful when working with acid as you could get hurt. If the buildup has worsened you may want to use a pumice block to remove the hard crusty parts. Once it has built up beyond that it is almost impossible to remove. Consult a professional as some older methods may damage the tile.