From what's been said, if the bag doesn't specifically say "Fire Retardant" then it shouldn't have any chemical and would be safe to use in aquaria.
So the one you have should be safe to use. Your's is already 100% polyester so you are good there.
But if you still have doubts, I use Poly-Fil made by Fairfield and it's safe. 100% polyester, doesn't mention "Fire Retardant" and I've been using it for years.
I get it Walmart, I prefer to get the batting/sheet/roll, not the loose fluff (for stuffing plush toys, pillows, etc.). The loose form just more likely has little pieces of thread that come off that can end up floating around in your aquarium. Haven't heard of it entangling and binding up impellers though. The roll is just more convenient IMO.