It was hard to photograph. the color is more of a light brown color and definitely less clay like in real life.

Some of the areas are porous but some of the places are slippery smooth. The patches were very smooth and even they are holding the "stain in some spots"
It was pure white at the end of last summer.

I tried vitamin C tablets both rubbing and leaving on.(no change)
I tried Iron out on a patch.(no change)
I took the water to Leslie's and they tested it and said no metal.

Last summer I had the patches redone because they were standing out from the summer before when they were patched. I also had him acid wash it to get the leaf stains out. They sanded and for the most part it really did look good. I had the same company plaster it a few years ago so he came back to "correct" it.

It does not seem to move with a metal brush. I think it looks like chemical to me? Is that possible?

I poured granular chlorine on the shallow end. (no change)