3" Black line around pool

Hi,

I'm a new fiberglass pool owner with a salt chlorinator. The pool was completed about a month ago and I finally got all the chemistry within the correct parameters. I keep getting a black line around the diameter at the water level. It's like a thin black film. I scrub it off using a scouring pad which is time consuming but effective. The black line will appear again within a few days and get worse every day until I clean it next. There has also been 2- 5" black oval spots that appeared shortly after the pool was completed. They are not slimy and are a few feet underwater on the wall. A scouring pad doesn't fade them at all. One is gone after putting the chlorinater on "Super Chlorinate" for about a week. The other is faded some. The chlorine concentration is reading 8 ppm but the level is probably much higher as the kit I have maxes out at 8 ppm. The high chlorine concentration seems to fade the black spots but doesn't fade the black line. I suspect the black spots may disappear if I kept the chlorine high for about a month but the water with high chlorine levels makes your skin burn and itch for a few days besides any other problems it may cause to the pool chemistry that I'm unaware of.

Does anyone know about the black lines that appear at wherever the water level is? Maybe dirt from the air pollution in the rain?

Thanks for any advice.