As DanC said, zinc plated fasteners and the white coating is "white rust" or Zinc Hydroxide. It is caused by exposure to moisture. It is technically damaging to the corrosion resistance, but as long as there is no red rust, and you remove the cause of the moisture, I personally think you are better off leaving alone.
This paper goes into detail about it. It suggests the only way to correct it is to use a scotchbrite pad and vinegar or a wire wheel and then paint with a zinc rich paint. The other option is to get them replated.
[ame="https://www.orrconsteel.com.au/sites/default/files/White-Rust-White-Paper-200531.pdf"]https://www.orrconsteel.com.au/sites/default/files/White-Rust-White-Paper-200531.pdf[/ame]
If you can't live with the white rust you can follow those directions or try EvapORust which you can get at Harbor Freight. I used it on some rusty lug screws on some old Premier bongos and it got the rust of without any further damage to the screws and washers. I just soaked them and scrubbed them clean with a brass brush. I doubt there was much plating left but so far they have not rusted back.