Mold from water leaking down through grout line
There should be a tile bead kit on the tub, if it didn't come with an integral one. That would keep water from going beyond the edge of the tub behind the wall.
How is water getting back there? Are any of the grout lines cracked? How long has it been since the grout was sealed? Is this on the side of the tub or on the front of the tub where the valve is?
Is it possible you have a leak inside the wall at the tub spout or showerhead that is causing small amounts of water to run down and buildup on the edge of the tub?