That's probably talc from soap scum mixed with the minerals from the water. Try dishwasher detergent - mix a cup in a bucket of warm water and apply. Leave on, then re-apply a few more times to melt the stuff. Then scrub off with a scrubbie sponge.

Best thing to do in the future? Don't use bar soap. Use only liquid soaps or if you must have bar soap, switch to glycerin soaps.

Kaboom is great for this and isn't toxic. Spary it on, leave it a bit. Take a long handled brush and scrub it around and rinse.

I use the Mr. Clean eraser on the shower glass after I clean the glass with Windex really good. This work's for me. Nancy

Here's some information that was posted in the Bathrooms Forum:
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Posted by: ownerbuilder (My Page) on Sun, Sep 11, 05 at 18:51

If you do end up with water spots, try A-MAZ Household water stain remover. I saw them do a demonstration several years ago at the state fair or some homeshow. The lady had a really disgusting shower door she was cleaning and it caught my attention. My husband gave me a hard time as I added yet another "wonder" cleaner to my arsenal of products.

However, it really works. It's easy and doesn't stink. The container says its biodegradable.
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You can get it at a local glass shop. I tried it and it really works...

TJ

You know how you are supposed to keep cream rinse in your hair for whatever amount of time designated on the bottle? I used that time to use shampoo and a 12 x 12" piece of dancing costume netting on the inside of the shower doors, rinse it off with a little splashing and that's that. I do this regularly so there is no big build up waiting for me.

Jois

After you clean the door, use Rain X on it. You can get it from the Automotive store. It's what you use on car windshields. I applied it to my glass doors and wipe the door dry after each use and I have not had one spot. the trick is to dry the door with a chamois or a terry towell. The rain x causes the water to bead up on the door so its a quick wipe with a cloth to catch the beads. Also, we only use liquid soap, never bar soap.

Hm. Good ideas. Will have to try some.

I've had some success w/ a scotch-brite scrubber on the doors, altho the tub itself is more difficult. May have to try brillo on it. :)

I bought some stuff called "Spot X" and the little scrubber that goes with it at one of the big box stores. It did a wonderful job.

My "glass" shower doors are in fact acrylic. I found a product in a ship chandlery in Newport RI called Fleetwax. It is manufactured by Collinite in Utica NY. It must be applied like Glasswax ( which did not work) so it takes a bit of elbow grease but it works great.

Watch out for the chemicals when you can get them close to a newer faucet. Could ruin.

Baking soda.

I ordered the EdFred glass shower cleaner shown on Muchfun's link and tried it. It took a lot of elbow grease and I had to go over the shower door and wall a couple of times. Applied it with a Scotchbrite pad. I can't believe it actually worked. It was 10 years of hard water build-up. I tried everything imaginable to clean it and nothing worked. I even tried, over the years, using a razor blade to scrape it off and that didn't even work.

I will be applying the Rain-X today and hope I can get DH to wipe down the door and wall after showering. Will also get some shower gel or use liquid soap. I sure hope that is the end of this very vexing problem.

I discovered the Mr. Clean bathroom & shower eraser a few months ago. This is after years of fighting this problem, and scrubbing, scrubbing, scrubbing with noxious chemicals. It's great! No chemicals, easy to use. It also helps to keep defeat the slipperiness of the shower floor. I often use it while I'm showering myself, waiting for my hair conditioner to do its thing.

Here is one trick to make them look better if the water spots still remain after you have cleaned them with everything under the sun. Put some lemon oil or Pledge on a rag and wipe down the glass. The oil hides the scratches. Make sure if you are using the spray can in shower to cover the shower floor with an old towel to protect it from over spray. Then make sure nothing (oil) is on the floor when you have finished.

Stove glass top cleaner mixed with a bit of Bartenders Friend. Rub till the water spot grittiness is gone rinse off. Does the job well.

I have just tried this method today in it worked wonders to get rid of my hard water stains. I used car polishing compound. Put it on a soft sponge and rub in circular motions then just wash it off with plenty of clean water and squeegee it dry. I put a coat of rainX on so that the water would just bead off. It worked awesome and after 7 years my doors look like new.