The stuff I was sold sounds like the cellulose material Waterbear mentioned.

It had a generic name like "DE filter additive". Instructions stated for DE filters or sand filter additive, and also stated it was 5x the efficiency of DE. It was off-white in colour and extremely lightweight.

It was $13 Cdn, for about 3-4 cups.

My question to the pool store where I purchased it was if it is 5x DE, how can so little of it spread entirely across the filter. She didn't know.