Persistent black algae can't be removed

[MonkaReel]

I have had this algae present in my pool for months and despite me shocking my pool and getting my chemical levels right (it's a chlorine pool) i can't seem to remove it. I attached two pictures link: https://imgur.com/a/mWSULWp

There is a before pic and the after pic is after shocked the pool then scraped with a bristled sponge. It seems like the algae is engrained in the actual pool and it seems stained.

I'm at my wits end here and need any and all suggestions for this.
Thanks!