Those pics help. I doubt it's a calcium scale stain because your CH seemed to be fairly low last I looked. Not that they are perfect, but the pool store test last Tuesday didn't show any metals. So perhaps organic staining from before when the cover was on. That's my assumption. How to tell? Well, even though the Magic Eraser didn't seem to work, you could try rubbing a chlorine puck around an area for a short time. Just don't press and hold it in the same spot, keep it moving slowly in a test area. This thread is getting lengthy so I don't recall if you tried the Vitamin C test just incase (iron), so you can do that as well if you like.
Did you use any copper-based algaecides last season or at closing?