Are they requesting you remove the hard water staining now?
If not, I would offer to do it when the glass panels are off, I would use cerium oxide and a makita polisher with a yellow pad.
With the panels off on a solid surface it will be much easier to remove the staining.
When polishing glass panels that are in position (fix in place in the stairway) there is alot of vibration and chances for it to crack.
Make sure to seal the glass panels after polishing, if not the hard water staining will come back fast and more severe.
Make sure that the panels “are glass” before polishing.