Spray them down with brake clean wipe them with clean towel, good to go.

Razor blade. Scrape it off, it’ll be fine.

You shouldn’t do your brakes with that question

Grind it off with a random driveway rock. Beers

Did it come like that? Can you not take it back to wherever you purchased from and ask for a different rotor or a refund?

It’ll be gone in the first brake application.

Did you try break clean

Lick it off. You don’t want to break a nail scratching it.

Looks like extra casting material. You can either sand that down with an orbital and 90 grit - or let your pads get rid of it the first time you step on the brakes.

Brake clean

Scotch Brite or steel wool. They make both in a scale of abrasiveness. It’s what they recommend to scuff rotors.

The brakes will kleen it

I would try with slick new scraper/spreader/spatula for wall works width 2 inch or so then by positioning it on rotor against residue at as small angle as possible.

like this one https://www.amazon.com/dp/B09FL49JP5/ref=sspa_mw_detail_0?psc=1&pf_rd_p=07d4f2af-6342-4249-a50e-928411266fe1&pf_rd_r=CWWWFPP2E94DZF1D4F8A&pd_rd_wg=JnjFs&pd_rd_w=zoscX&content-id=amzn1.sym.07d4f2af-6342-4249-a50e-928411266fe1&pd_rd_r=8810baac-ed38-4402-bc52-ab3d943ac8e9&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9kZXRhaWxfdGhlbWF0aWM&th=1

If brake cleaner doesn’t do it and scrapping it isn’t effective. You could get them turned. Or do a return

Use brake cleaner. Spray can at any auto parts store for $5-$8. That residue will stay on your brakes pads themselves and leave a coating (very very thin but still a coating) on the pads and cause your brakes to squeak a lot earlier than they usually would from normal wear. 👍