As above, that's standard flash corrosion and all cars with steel brakes do that.
If the NSR sticks on permanently, different story. You'll know when that happens because at the end of your journey, your nose is distinctly aware of eau de brake dust, and that wheel will feel hotter than it's opposite.
And no, in that situation, don't bother with a refurb kit unless the piston is totally mint. If it's corroded and pitted, you'll be wasting your time and money. Just get a new caliper.