What special products are required for cleaning marble showers?
You would want something that is made for or says "safe to use on" marble and stone. John Koessler over on the John Bridge forum (a tiler and also a rep for STT products) recommended STT's cleaners EZCLN and HDCLN for my marble, stone and tile--I'd called him to discuss and order their sealer and we discussed cleaners as well (I'd run out of SCI's Marbalex, which we had been using, and can't find it locally any more--if I had to mail order something anyway, it was worth trying the STT cleaners since I was already ordering from him). My Thassos mosaic in the MB is in the process of being installed, so I haven't used anything on it yet! I can say, that I tried the HDCLN on my discolored grout/glass tile backsplash in my kitchen (just above the vent trim at the back of the range), and it worked very well--faster and easier than using OxyClean.
I had called STT directly a couple of months ago while I was doing my HW/research on sealers (hated the stuff we'd used in the guest-bath), and spoke with Steven, the owner of STT. Really nice guy (as is John, BTW), and passionate about their products; their effectiveness and safety, etc. Patient, too! Both men have answered my numerous questions and gave me tons of info about the products, their uses, and any other marble and stone questions I had.
I'll let you know how well the cleaner(s) work on the marble when I get to start using the MB!
I bought Miracle Sealants Tile and Stone Cleaner. I've had the new marble tile about 3 months and it works great. You can get it at HD.
Here is a link that might be useful: Miracle Cleaner at HD