Interesting discovery

Suggest you treat solid conductor cables very carefully (once set up....don't move them at all if you can avoid it).

Most cable problems come from slightly broken, damaged or oxidized contacts - leading to high or intermittent contact resistance on conductors and/or reducing the shielding performance on shielded coaxial interconnects.

Braided wire, although slightly more expensive to manufacture than solid wire, is good for flexible applications such as speaker wires. Braided can be soldered more reliably and is much less likely to be damaged from flexure.

Since oxidation from copper would be green, might the white powder simply be excessive flux from the soldering during manufacturing. Excessive flux should not do any harm, provided you have enough of a well soldered contact area with the spade.

Yes, the powder is oxidation, and is not something you want to happen. It will degrade the cables ability to transfer a signal. It might be a good idea to have the cables reterminated with something that will fit your speakers.

Oscar Wilde also made an interesting discovery. He said "I have found that alcohol, consumed in sufficient quantities produces all the same effects of drunkeness." I hope that helps.

I can't say what the powder was, but if you're seeking to maximize your connections, please consider a contact enhancer. I use "Walker extreme SST" with great success.

White powder is not from copper oxidation. Copper oxidation is green.

As Yohjo above. The powder is probably there to absorb humidity. Of course you've cleaned the wires thoroughly before connecting.
As a side issue, you can leave the wires as is. You'll have to clean them from time to time (the oxides) and you can use some chemicals that promise to slow down oxidising (Kontact and others).