Every time I've used the red sea nitrate kit, it always has that little bit of precip in it. I was told by Red Sea its completely normal, and if you watch their youtube video's on how to use their kits, they also have it.
Its just how they work, and will not throw off your reading at all. If you "shake" the vial again after the 9 minutes like they say, most of it will mix in and dissipate, but there is always some.