There are a lot of feeds on the market that contain DE in them already for this exact reason.
I don't know how much feed you have left but if you put some in a gallon size Ziploc bag I would sprinkle a few sprinkles of the DE in there and see what happens if you left it overnight.
Grain mites very different than the mites that suck the life out of the bird.
There's no reason why you should not or could not feed the green might infested feed to them.
You can sprinkle DE in to the feed If it is completely dry then the DE should kill the grain mites.