I'd say it looks like the water absorbing crystals that are in baby diapers, also in some planting mixes (Miracle Grow Moisture control).
This stuff would be jelly like when wet, firm but squishy, then drying to small crystals about like coarse salt. There are various grades of it, fine (like table salt when dry), medium (like Kosher salt), and large (like rock salt). I know that when my kids were small, I would take them to the neighborhood pool in diapers and when they got wet, the "super absorbent" type which had that inside, would balloon out until it was about to explode as the crystals absorbed something like 100 times their weight in water. I have bought it by the pound to use in landscaping or planters and I've also used it to sew into neck coolers to sent to troops in Iraq.
But then, I've never used any flocculent so I wouldn't know if that would look like that.
[edit] I suggest sweeping it up, then wetting it again. If it swells up, probably this hydro absorbing polymer stuff. Then take a long look at what babies have been in the pool in diapers. I suspect that the "swim diapers" are made without that stuff, just guessing. If no babies.... anyone use those "neck coolers" in the pool? Your pile is way too big for a neck cooler, each holds only about a teaspoon of dry crystals, but softball sized is about right for a baby diaper.
And yeah, if a diaper blew out and exploded in the pool, one might not see it, as the stuff is pretty clear when fully expanded, but it would make a huge mess in the cartridge and would tend to pack it so much that I would expect it could break the bands due to high pressure. I'd guess that filter pressure would climb so quickly that one might not notice in time.