A slurry is generally a term for CLSM - controlled low strength material, which is a form of concrete. These materials flow like water so it will fill all the voids, cracks and crevices below the pool without having to compact as you would stone. Additionally, it has a low compressive strength - like 50-100 psi whereas typical concrete is around 3,000 psi strength. With this low strength it can be easily chipped out or removed if necessary, say a plumbing issue. Think of it as a liquid sand mix that will bond together once cured. It's typically used to backfill an excavation when you want as little settling as possible and I would think it would be a better backfill material than crushed stone because of the difficulty compacting the stone in tight places in and under the pool.