Any chance you could upload a photo? Close in and far away views would help. New work or old? Any recent renovations done, plumbing work, etc?

Probably the most common complaint is orange staining, it's caused by the oxidation of iron deposits within the marble. Second tends to be blotchiness from installing the marble with mastic instead of thinset, but that's usually a decorative backsplash thing versus a shower thing, as mastic really shouldn't be used in a shower.

New marble tile work may need a week or three to dry out completely, the moisture in the thinset needs to fully dissipate. Until then new installations can look blotchy.

Some epoxy grouts can penetrate into the edge of the tile, causing a stain or discoloration around the perimeter of the tile. Usually referred to as "picture framing".

Improperly applied sealers on shower floors installed over a deck mud base can bugger things up big time. If too much sealer is slopped on it can penetrate into the mud base below the tile and limit water movement through the mud, causing the mud to hold water. You can end up with a blotchy looking floor that never seems to dry out.

Marble? I love it. But it can be a pain at times.

That's all I have to say about that.

For now. lol