Pool: In ground, gunite, white plaster, salt system.

I've had a 'sand' problem ever since the pool's first year, about 3 seasons ago.
A brown gritty substance collects on the steps and the outs of the pool. Usually I can brush it off, but it always comes back the next day.

Well, not it's not brushing off as easy as it used to. I have what looks like a brown stain on the plaster on the steps and the outs (any flat surface the polaris can't get to).

I rubbed a lemon on it for a few minutes, I assume it technique is supposed to be similar to placing a Vitamin C tablet on the stain, but it didn't change the stain at all.

I don't think it's algea as it's not slippery.. it's gritty.

What would be a good technique for trying to get this off the plaster?