I have tried everything I could think of to clean a fiberglass shower pan. My 87 year old mother was not able to clean it. Since I moved up to NY and into her house, I have tried to remove the gray residue. You can't use just anything or it will destroy the finish, so always read your cleaning product to make certain you can use it.
I decided to try those cleaning sponges, but didn't want to spend extra if they didn't work. So, I bought the generic brand in a 2 pack. It took a lot of elbow grease, but it worked. Get a large pack though, you will need it.
I have tried it on tile, but am not sure yet whether it worked or not. There was a cleaning product that I used where soap from the soap dish accumulated on the tile below the dish. It worked very well, but until I try it on the rest of the tile for the hard water I won't mention the brand. I'm hoping it works because it was very economical compared to many cleaners. Just tried this a few minutes ago, so more will be coming.
By Patt from Grand Island, NY