If it's stained, the discolouration might be permanent. Grout should be sealed, otherwise it will absorb moisture (along with any colour from the mildew). The sealant will wear off eventually (from cleaning and just constantly getting wet) which is why you might not be able to get it to look clean.

I've had success with a grout whitener (which is just white paint made for re-whitening grout) which I've then sealed. I've only done this with ceramic tiles, so make sure this works on marble grout.

If that fails you can also use a dremmel tool to remove the top layer of grout and re-grout. But be sure to seal it, Carrotsticks is right, it will discolor if it's not sealed.

I'd suggest half/half vinegar or peroxide and water. Both of them do a pretty good job, but you may need to scrub some areas. You could also make a paste of baking soda & a little water and scrub with a toothbrush.

That does look like marble to me, too, and the staining looks like mildew. If it is, strong chlorine bleach-based cleaners (and elbow grease) can kill and clean mildew, but I'm not sure if that's safe for marble. (I would imagine so, but "imagining" is not knowing!)

If nobody here can confirm a method, I'd call a tile installation company and see what they advise.

I saw a tip online that steam is an excellent grout cleaner. I don't have a steam cleaner, but I I have one made for steaming clothing. I tried it, and it made a huge difference. I steamed a small area, then scrubbed lightly with a toothbrush. While the results are still not perfect, the improvement is huge. It was way less effort than my previous attempts involving various chemical cleaners + scrubbing.

I'm a Dremel girl! And I have a steam cleaner. I'd go for Dremel on that grout and then seal it.

someone told me to try oxy-clean on my grout once, and it di dwork for a time...

if you don't want to invest in a steam cleaner, try running the shower with hot water for 15 minutes and then scrub with a toothbrush. the steam from the shower helps loosen the mildew almost as well as a steam cleaner would!

Please don't clean marble with vinegar, lemon or other acidic substances. They will harm the marble.

I would NOT use vinegar on it if it is marble. I once made a mess of a granite countertop making lemonade--the acid in the vinegar will damage granite and/or marble surfaces.

Once you get it clean, use a PENETRATING SEALER, not one that lays on top. Tilelab makes an excellent penetrating sealer. Apply per bottle directions and apply enough coats so that water beads up on the grout. After several months you will notice that the water isn't beading up any longer. Reapply until it does.

The easiest way to avoid this in the first place is to buy a good quality silicone squeegee and squeegee your walls and doors after every use. Leave the doors ajar for good air movement.

Good luck.

Do not use anything other than a cleaner specifically made for natural stone or you risk discoloring, etching, or wearing away the stone. Check out Mapei and VanHerron cleaners for natural stone (google it, you can find them at tile & stone dealers) and they will help get your grout cleaner. It's unlikely it will look brand new but it'll look better. And then make sure you seal the hell out of that grout & stone and only use natural stone cleaners from here on out!

I just bought Bona, a cleaner for marble tile. I used it to clean my marble tile floor this weekend. It works fine.

I'm also a fan of the squeegee-ing after the shower thing. I renovated my bathroom with marble tiles a year ago, and I havent had to do any cleaning on the walls the entire year - I think it's because I squeegee everyday.

I would call or visit the closest Tile Shop and talk to those folks.

I tried off-and-on for years to clean the grout in my last shower. I tried vinegar, bleach, oxyclean, grout pens, Mr. Clean magic erasers, ... eventually I just cut away the old grout and re-grouted. It was much easier than I expected it to be, and I wished I had done it earlier.

I was in this exact situation a month ago! I followed the instructions here. and found that they worked really well. I also ended up having to re-grout some areas, but that was due to cracking, not discoloration. Now I squeegee the tile after every shower and it seems to be holding up well.

csmcgn: I read great reviews of StoneTech Bullet Proof Stone Sealer, so that's what I chose. No issues so far, but it's only been a year and I squeegee'd the tile/grout after every shower. www.amazon.com/gp/product/B00065W8G8/ref=oh_details_o02_s00_i00

My favorite grout cleaner is the Magic Eraser--it's great for soap scum, not sure about mildew. I'm also not sure how it will treat marble. I would guess it will be fine, but I would test a less visible corner first

Chip away the top layer of grout and regrout and then, as already suggested, seal it.

I wouldn't recommend bleach. I once tried this on a corner of my mom's marble bathroom, and the marble loss its marbling. It took the bleach only a few seconds to turn it pure white. Thank goodness, it was only at a corner, so my mom didn't notice. (Or at least she didn't bring it up if she did notice).

Marble paste from ENJO is the only solution I can think of. Use it with a hard brush and will definately make it better!

I used NuGrout Colorant to color the grout in my kitchen. My floor looks fantastic! The best part is I saved a bunch of money since I didn’t have to re-grout the tiles. My wife LOVES the outcome! You can find out more about these products at http://www.nugrout.com.