Custom Milling sells Perma Guard Fossil Shell Flour (their name for it). It's great stuff. If it's very bright white, it's usually the right stuff. If it's gray, I'd hesitate.
This does sound like Food Grade. The percentage is correct anyway. Does it say anywhere on the bag "Food Chemical Codex Grade"? The less than 1% is the right type. Pool Filter grade is about 85% silicon dioxide.