Same here. I also ruled out the filter sand the same as you did.

The stuff is so fine that when disturbed it disappears for a while till the pool settles out again.

At first I thought it was metals precipitating out of the water from when I first filled it....a magnet doesnt help....but maybe its copper, or gold!

I also thought maybe it was from the yard but it doesnt accumulate daily...seems to be the same once settled as when it first appeared.

Even my dolphin cant filter it out...it just tosses it around and the next day its back.

So I guess we're both stuck with it unless someone has a better Idea of how to get this stuff out of the pool.