Aluminum oxide is an adherent oxide that will always form on aluminum. As a coating it is tenacious and chemically fairly inert. Something major and extremely out of place would have to occur for it to flake off as you describe, so to me this is an unlikely event.

If you have sheet metal ducting, it could simply be zinc oxide. Same stuff that comes in tubes of creams for insect bites, etc. That's the oxidation product that forms on galvanized steel when corrosion takes place. I would look for moisture getting into the ductwork and corroding it.

Thanks for the reply. My duct system is not Aluminum but flex duct. Could flex duct corrode? What about aluminum tubing leading to the end of the coil? If that is the source, would I also be able to visual see it in the pan or on the coils?

Duct systems are never aluminum, way too expensive a material to use for ducts! Ducting will be either flex duct, galvanized steel sheet metal, or fiberglass rigid ducting. Any sheet metal distribution boxes in the attic for those flex ducts? In a humid environment, those boxes being chilled by A/C could have water condensing on and/or leaking into them?

If I weren't a retired metallurgist, I'd say send me a sample of it. A local metallurgical testing lab with an electron microscope (or a local university) would be able to tell you in 5 minutes what it was. Honestly, I don't think its hazardous, only a nuisance for you.

We were told its coming from hvac unit. we hare condensation in the pan in attic under the hvac system. when the heat comes on in winter conditions, the water that evaporated leaves a residue in pan and on coils, that blows out the first few times in heat mode.