My pool liner has a dozen stains which weren't there 6 months ago. I have picked at them with my finger, and can feel no difference from the non-stained areas. They are a mid green color and are grouped at three sites.
My pool water has remained mostly crystal clear the three years I have owned it. My water checks fine. There is only occasional minor tree debris on the bottom and the vacuum is on constantly.
No little kids with dirty feet, etc.
Any ideas of what they are, how to clean them and prevent more?