That looks like, as DawnDenise said, efflorescence. That leaches though (or "flowers" if we want to be true to the root of the word) from the concrete/gunite, as opposed to being deposited by your water. Not much practical difference when it comes to removing it, but all the water chemistry in the world won't prevent it if it is efflorescence.

This is pretty common, as I understand it, on raised bond beams and exposed gunite walls. I get it on the back side of the negative edge, even through the tile.