Try a paste of hydrogen peroxide and baking soda and let set 20 minutes. It may take multiple applications. Hydrogen peroxide turns to water after exposure to air about 20 minutes.

Wiping tub/shower after each use with squeegie or chamois will tend to keep surfaces dry and minimize mineral deposits and mold/mildew. Leaving shower curtain/door open will improve ventilation into the tub/shower area. Keeping surfaces clean and dry will minimize mold/mildew as will improving bathroom ventilation. Run vent/fan for 30 minutes after bathing to help remove moisture from the air in the bathroom.

The stain was caused by bleach in the mildew cleaner. The bleach reacts to the porcelain when the protective finish allows the bleach to react to the cast iron underneath.

The solution is Clorax 2, the stuff you use for laundry. Pour some on the stain and let it sit and then scrub. Never use any bleach type cleaners on the tub again, because it becomes increasingly harder to get rid of the stain.