Tiles Hard Water Stains

use spray sanitizer for small areas and see the result.
It removes any kind of strain oily to hard

Soak area with apple cider vinegar / vinegar for sometime and then wash with soap & old toothbrush

Get bathroom cleaning chemical/acid (hardware shop, you need chemical for cleaning toilet pot basically)
Spray it..it will be all shinning back in no time

Harpic Bathroom Cleaner 

Call from urban company if money is not issue, they will take care of these things

I am also facing similar problem with my glass wall panel. Lot of stains on it due to hard water. It is mostly basic in nature so mild acids should work on it. So vinegar mixed with liquid surf should work well on it not sure though.

Let me know if anything work for you. Will follow same in my house too.

i clean my bathroom tiles stain with water mixed acid(locally available 5 ltr bottle as bathroom cleaner) and add some floor cleaner in it..use some scrubber with handle on it. just make sure its not concentrated else it can harm marble on floor and then clean with water some part that fell on corners also cleans marble floor. i wear rubber gloves while cleaning.