I just purchased a home with a 1960’s -era pool. I am new to pool care and am looking for a way to clean the black buildup that has accumulated near the waterline. What is it and what are the most effective means of removal? So far I’ve tried muriatic acid and RMR-86. I had some success with muriatic acid but, as you can see in the pic, the most stubborn sections remain. I’m assuming it will take a combo of muriatic acid and elbow grease, but am not sure what type of brushes, bricks, etc. to use. TIA!!!