That's not a stain, that is the fiberglass or acrylic having changed color. Not getting rid of that.

I agree that the exposed has faded from yellow to cream. I’m rehabbing A 1950 cottage with a type of fiberglass shower surround which is an olive green. Upon removing the glass shower door track, the panel underneath was discovered to originally be a lovely aqua blue. Major atmospherically influenced changes!

Try the toilet cleaner “It Works” or “The Works”. It removes hard water stains very well.

That’s not hard water. Acidic cleaners will just ruin the plastic even more. That’s toast. Replace it.