What should I use to clean a salt cell?

Use muriatic acid. Soak the cell in a 4:1 water-to-acid mixture for 10–15 minutes to dissolve calcium scale. Kill power first, mix acid into water, wear gloves and goggles, rinse thoroughly. Don’t use metal brushes or scrape plates, and check your manual — some cells are self-cleaning and acid soaking voids the warranty.