By Jeanne Huber - Washington Post
Q: The marble threshold for my shower has reddish marks. How can I remove them? Annapolis, Md.
A: Marble can develop reddish marks for a variety of reasons, such as spilled red wine, bacteria or moisture that interacts with the iron in the stone.
In a shower, you can probably rule out wine or other spills. But bacterial growth is definitely one of the prime suspects. Serratia marcescensforms colonies that look pink or red on surfaces that are left damp for long periods — and bathrooms make an ideal habitat. If it is allowed to spread, it can cause illness in humans and pets.
A pink ring around the waterline in a toilet, pink grout lines on a tiled shower wall, or red slime on the rim where the shower walls meet the shower pan all can be from this bacteria.
It’s usually possible to just scrub these colonies away with soapy water and a bristled brush. An old toothbrush works well in crevices. But when natural stone, especially relatively porous marble, has been etched with acidic cleaners or is not topped with a sealer, the bacterial growth can extend into the pores of the stone and be impossible to scrub away. Here's the link to the full article: How to remove stains from marble shower tile