stains from iron in the stone is essentially surface rust. It would be orange/brown in color. I don't see anything like that here.
What you have is simple water penetration. It could be from a number of reasons, but the most likely is that it was sealed enough.
Grout needs more sealer than the stone. So if the grout isn't sealed thoroughly, water will make it's way through the grout and into the rough, saw cut edges of the tile, where it is far more porous than on the face. Also, because these pieces of marble are so thin, it is easy to penetrate and saturate the whole piece, instead of just darkening the edges of the tile like you would see on full sized pieces.
I would recommend starting this way:
1. Clean the floor thoroughly and dry it.
2. Let it air dry as long as it needs and keep track of how long it takes. If it takes several days, so be it. Use another shower.
3. Seal it well, pay special attention to the grout. Do not apply an additional coat of sealer until the grout and stone have returned to their natural color. Pay attention to how long the grout stays wet when you seal. It should soak in less and less with each application. I would say the grout will need at least 3. Re-apply until the sealer just sits on the grout for a while.
4. After your last application of sealer, wait 24 hours, then apply one last application.
5. 24 hours after your previous application of sealer, you can use the shower.
This is all slightly overkill, but you have porous, very thin strips of marble. Take the time to it right and I think it will be okay. IF this does not work, you may possibly have issues elsewhere.
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