Stains that appear and disappear

[RayFaye]

Hi All. Thank you for taking the time to answer my question.
Our house at about 100-120 ft from a lake in upstate NY. We have a salt water pool that was in place when we bought the house. The pool is 12 years old.
For the past 5 years the pool liner has been stain free. Some spots are white but I believe they were white when we moved in.
Around 2 weeks ago, I noticed the pool was severely stained (seems brown). The pool maintenance crew said that the chlorine and salt were at a perfect level as well as the pH.
It seems to be that when the pool cover is open, after about 20 minutes, the stains start to appear. When I leave the electric cover closed overnight and open in the morning, the stains are gone. Pool temperature is around 84.
With regard to the stains, the floor feels fine. It’s not gooey or dirty. And there is nothing to brush off.
What on earth is going on?

(Is there a way to post pictures on here)