3M/diamond brite has brown flecks everywhere! Acid wash/soak?

[fox_and_hound]

My brother just got his first house, the pool was green and growing all sorts of yuck. So we got it up to speed and water is clear and balanced now, but the plaster granules/3m/diamond brite looks damaged. It has so many flecks of brown everywhere. About 50% of the flecks are brown. I am assuming they used to be blue in color because there are so many. I don't think they are intentionally brown bc they are sporadic. Are there any options here?

Anyone ever see this type of damage- any remedy?

 

[JasonLion]

Lets try to narrow down what might be happening. Try placing a vitamin c tablet on some of the brown flecks and see if that makes any difference.