I just did a google search on Terra Marrone and came up with a glazed porcelain from the LeArgille line at Casa Dolce Casa. Linked it below. If that's it you could try calling the manufacturer. If not, I would suggest trying something a little milder than bleach first, just in case. You could try a mix of baking soda and vinegar, or a little lemon juice (as long as it's not actual stone). That would also be better for the environment, and it won't asphyxiate you either! Or you could try one of the off the shelf mildew cleaners, but be sure to open a window & turn the fan on -- the fumes can be strong. Good luck!

Here is a link that might be useful: Terra Marrone

In my old shower prior to tear down, I had phenomenal luck with Mr Clean magic eraser. But test a small area first. I don't know what it's made of, but it was great on the grout and tile (ceramic). It took some elbow grease, but I didn't add any chemicals (I don't know and couldn't tell if there were any in that product).

I found an inexpensive peroxide-based cleaner that works pretty well on our travertine shower for pink growth on the grout. Oxyclean might work also (I'm admittedly loath to recommend the stuff b/c it's 'as seen on TV').

Here is a link that might be useful: Betco peroxide cleaner

Buddysmom: We have exactly this tile in our master bath which we had redone two years ago, and we got it from Expo too. We love it. It's definitely porcelain, and I clean it often with Chlorox bleach cleaner, the spray on kind, because I hate mildew making an appearance. So far, the tile has held up beautifully. It can sort of take anything, from what I can see. Good luck. Hope this helps.