White Vinegar or lemon juice can work and a non metalic scouring sponge.. However often with hard water stains the glass becomes etched. Once that happens nothing will clean it.
My shower had a similar problem, the glass needed to be replaced, to look crystal clear again.
Google hard water stain removal. There are a plethora of articles out there.
I use white vinigar! I have so far, not encountered any hard water stains that will not come off with vinigar. Sometimes I hve to let it soak overnight though