Crema Europa has tiny little pinholes in it. If you look closely you can see them and some are 1mm deep or so. They appear in clusters. If you look and inspect this stone, you will see some areas are porous, while others are very smooth. The area with the mold is one of these pinhole clusters. If (WHEN) the water absorbs into the stone, it can sit there overnight, even with ventilation. If it is a steam shower and they shut the door after using it, the water will stand for a long time and develop mold.
For starters, I would recommend MB-9 mold and mildew remover:
http://www.easystonecare.com/MB-9-Mold-Mildew-Remover-p/mb-9.htm
If you want to take it further, this will work for removal, but can also be used as a preventative treatment:
http://www.easystonecare.com/D2-Biological-Solution-p/d2.htm
The only way it would be coming from the back side would be if the drywall or studs were wet. I doubt there is water proofing up that high and you would see patterns much worse than you see here.
I have worked on Crema Europa probably more than any other stone. It is actually very easy to deal with for a limestone. I think you should be able to clean this up and make this customer happy.
I would recommend sealing with F-721 mixed with distilled water:
http://www.easystonecare.com/F721-Concentrated-Stone-and-Grout-Sealer-p/cc-f721.htm
Give, or sell the customer the MB-9 so they can periodically deal with it.
If you don't want to bother with any of the above, mix bleach 50/50 with warm water and clean it up.