If I remember correctly it's a powder to prevent oxidation of the wires. Basically a protective coating to increase the lifespan of the wire and prevent oxidation of the copper wire and prevent corrosion of the wire.
It's nothing dangerous and the powder will wash off your hands if you get it on your hands.
If it's not that then I've seen some corrosion in older cords and wires so it could be corrosion turning the copper to powder.