It could be diatoms but if you can just blow them off I doubt it. Mine require rub with a piece of cloth to remove. I have Floramax substrate, didn't bother to rinse it, and if I stir it up it does get a bit messy but settles in 10 minutes or so. Of course all that dust then sticks to the diatoms for double dose of fun.
If after you blow it off and it's back a week later without explanation it could be diatoms.
If after you blow it off and it's back a week later without explanation it could be diatoms.