The first time I drained the pool was because no one could identify, let alone tell me how to treat, the most bizarre stuff I've seen in and body of water.
This was the "housewarming" gift from the mortgage company - under the 15.000 gallons of green there was a solid brown sponge-like covering about an inch thick coating the entire bottom. From time to time, it would release a chunk of itself which would float to the surface and dissipate after a few hours or days. My guess it was its reproduction scheme - send a colony to float downstream and expand itself.
I dumped the Leslie's suggested "Shock". which removed the green, and turned this stuff white. but did not break it up.
Here's where it got weird - as the water got below knee heights, I used a push broom to sweep the crud toward the drain. It actually rolled up like sod - and picked up the crushed stone which had gotten into the pool over the years.!
Anyone know of a life form which would roll up crushed limestone like that?