I went to clean my 16 x 32 vinyl lined pool and found I couldn't even brush the growth off. I originally thought it was just dirt laying in the low spots but even with vigorous brushing it does not come off. It appears dark to black at first then turns yellowish after brushing. I gave it a brush last night and threw 3 litres of 12 % in but this morning still is not letting go. Going to try to attach a pic.