Black Algae?

[bobmano]

I seem to have black/brown patches on the side walls. The water itself is crystal clear. I see these patches for some time now and slamming doesn't seem to help. I'm not sure if its black algae or the surface itself is damaged. Attached some pics here of the walls. Any thoughts on how to get rid of this?

 

[Donldson]

Have you used any kind of algaecides in the water?

 

[bobmano]

Yes, i used algaecides a couple of times this year and also phosphate removers last month in addition to slamming

 

[Donldson]

Would that be a copper based algaecide, by chance?

 

[bobmano]

It was from the pool store. I thought it had copper, but when i checked just now, it says free of metals