Pea sized black to rust colored spots

[bizbad]

For two years now I have had two tiny spots in the shallow end of my pool. They don't get any bigger and they are not multiplying so I have been lazy in fixing the issue. I did try scrubbing them and applying vitamin c to no avail. I am wondering if it may be rust? I had a similar spot three years ago and the pool builder scraped it off the gunite with a screwdriver. Would scrubbing the spot with ph down granules work to see if maybe it is rust? Or what else could it be?

[waterbear]

If it was rust then vitamin C would have removed it.