CLR cleaner or Lyme Away. Use gloves & lots of ventilation. I’d scrub it with a nylon scrub pad, don’t get any on your face (splashes).
It May take 2-3 applications.
I use "Whink" rust remover for almost anything that needs rust remover. You squirt it on the area and watch as the rust disappears. Just keep it off of gold/brass colored fixtures.
I use Bar Keeper's Friend to get rid of rust stains. For really tough ones, I make a paste of 2 parts Bar Keeper's Friend and one part water. I cover the stain with the paste and then put a piece of saran wrap over it. That way the paste won't dry out before it has time to do its job. I let it sit for an hour before I try to scrub the stain away. Make sure you use gloves.
You could try these all natural methods: https://www.hunker.com/13423502/how-to-remove-rust-stains-on-a-shower-floor