Amateur Pool Guy

I’m going with the pollen answer. I have a 20k gallon in ground pool. We have a screen enclosure, and our back yard is surrounded by pollen baring trees. We don’t use the pool that much, nor do we use sunscreen. It’s funny that the only parts of the pool that attract this grungy black stuff are the top of the floating plastic thermometer, and the inside of the plastic skimmer around the water level line. I haven’t tried the Magic Eraser yet, but I plan on doing that soon.