I imagine the white specks are perlite?
The photo of the back of the bag; where it states what it's made of, lists perlite as an ingredient.
I'm not sure, but I think perlite is mineral. not sure if it's one of those things where they pump the rocks with water and then heat them until they popcorn, or if it's actually naturally found in that state, but I'm pretty sure it's rather inert. No clue on this, but I'm wondering if it might have decent CEC or something if someone managed to get it waterlogged (Sisyphus may have some tips on accomplishing this...).
But, yeah, while I don't think they would be harmful to the aquarium, good luck keeping them in the substrate. Plus, I think the main reason they are put in mixes is for aeration or lightening the substrate, which I don't think is much of a concern for aquatic substrates...