My 60lb bag says "Calcium grit" -- which is not the same as grit for grinding food.
Oystershell is the shells of oysters specifically, calcium grit is a calcium source not necessarily from oystershell (honestly, it looks like limestone gravel), and grit is granite for grinding.
They need both...
It's better not to mix it in. If they need grit they will take it, if they don't need any right now they won't.
Since it's granite it takes a while to wear down inside their crops so they only eat it intermittently.
The caveat here is that in many, if not most, cases such feeds are better fed fermented or, at least, wet, in order to make it more difficult for the birds to pick out their favorite bits and give themselves nutritional deficiencies.
Welcome to BYC.
I have nothing to say in re: the "clean" issue, but many people here recommend using an all-flock type feed in the 18-20% protein range with oystershell served on the side instead of the common 16% layer feed that has calcium included.
The reasoning being that said layer feed...