Stuff that chlorine oxidizes is essentially 'gone', so far as filtration and settling goes, unless that stuff is iron and it's not in your case.

Brown stuff at the bottom of the pool is often dead algae, but from what you've said, it doesn't sound like that's happening either.

Gray stuff at the bottom, especially in a clear pool with a DE filter is often . . . DE.

And that means either a leak in your filter (torn 'membrane', missing tube, misplaced connection, cracked header) or an error in your filter startup process.

If there's enough stuff to pinch, grab a pinchful: DE will be a little gritty.

If it is DE, your filter OR your backwash process needs some work.

PoolDoc