These photos show the side of the pool at the top 6" or 8" or so below the bottom edge of the tile line. You can see in the 2nd photo that the surface is not only stained, but fairly rough. I have tried an acid wash and also scrubbing with other heavy duty cleaners, but with no affect whatsoever.
I should also note that since the pool has been filled and maintained (after these photos were taken, the stain looks much more greenish than in the photos, which look more brown/grayish. It may just be that the water gives it the greener tone, just like the unstained parts of the pool look blue once water is added, but white when it is empty...
I was hoping that it would fade after the pool was maintained for a while with chlorine, but it has not changed at all. As you can see, there are small pieces of plaster that have chipped off, and those are the color the plaster should be, so I am thinking of trying a very heavy/hard wire brush on the surface to remove it, but that could be pretty hard on the plaster. I did try a brass brush, which is much less aggressive, but that did nothing.
I am hesitant to try use a stain remover since I have been trying to avoid introducing new chemicals to my pool that would throw off my balance, but I'm not sure how else to get rid of this stain. I know the pool is clean and balanced, but it is a little embarrassing when we have guests over to use the pool.
Thanks for your input!