No matter what it is, you need to have those ducts cleaned so you don't get more of it in the house. Looks rusty to me, BTW.
Try to pick the flakes up with a magnet. If it is rust, I'd assume it should be attracted to the magnet.
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Woke up this morning to these little brown flakes coming out of most of our AC ducts. Google tells me it's probably rust or mold. Can anyone tell by looking at this? Trying to decide how freaked out to be... Thank you