Hard to say if that's algae or a stain. If you scrape some of it off and smear it on a white piece of paper, then if it's green or dark green, then it's black algae for sure. That would normally only grow in a pool with proper FC/CYA levels if the circulation in that area were very poor. You'd also want to completely get rid of it at its roots by scraping it off and exposing it to high chlorine level. A Trichlor puck would do that, but would also likely leave a mark on your pool surface and you've got such a nice looking surface I wouldn't want to ruin it.
They were not there before they just started showing up, when I had a pool man taking care of it before TFP they were not there but the FC was at 16 and CYA was over 100. When I started to take over I added the swg..then since March 2014 they started showing up..They look like stone, but what made them show up...Thank you
Merged by moderator. Please keep all related questions discussion in the same thread. jblizzle
When I took over the pool from the pool man, I had no algae, at that time FC was 16 according to pool store. This all before getting my test kit and TFP.
Now the pool FC has never been below 5, and balanced... Now I have started getting black spots started with 1 or 2 now about 10, just trying to see if I did not take care of pool right. I have been steel brushing algae spots...The pool does look great except these growing spots.