Looks like moisture. White marble always gets dark when it's wet. The first step would be to find where the moisture is coming from and then resolve it. Then try to dry it out and hopefully no iron staining will result.

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> On Oct 6, 2016, at 11:24 AM, Justin Cameron <justin@jmcstoneandtilecare.com> wrote:
>
> Hi folks
> I got these pics sent to me from a designer who's new install shower has discoloring. I haven't seen in it in person but just wanting some further input on why and a solution
> Thanks again
>
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> Sent from my iPhone