Rust happens. It’s not preventable or curable. The iron is inside the marble.

I have marble tiles in my shower - if and when they look grungy, I will hire a professional grout/tile restorer company. I have seen what they have done for stone in the public areas of my building.

It's a shame that you have pooling water at the drain - my shower slopes quite a bit - and markedly so in the area right around the drain and there is no water at all when I exit. I know because I use my hand held to make sure there are no sudsy bits on my floor.

After it has been restored, you might use a wash cloth or sponge on a stick to mop up any small pool of water as the last thing you do before exiting the shower to prevent it from happening in the future.

I have the same issue.
What do you use to clean the grout so you don’t hurt the marble tile?
Thanks and good luck!

Hello - I'm reviving this thread to ask if you had any luck with your research and to see if anyone has experience with Black Diamond grout cleaner. I have a bottle of it and a grout brush ready to tackle a couple of areas on the basketweave Carerra marble floor in our main bathroom, but I'm still nervous about etching and came to research on Houzz.

Unfortunately, our contractor used the white grout we specified for the walls instead of the gray we specified for the floor and being new to this, we let him leave it. We regretted it quickly and wish we'd made him change it out.

Thanks!