I wouldn't worry about that too much; although washing the salt off is a good idea.
If you're feeling ambitious in the summer you can take it apart, sand it a bit and paint it, or just order a new one.
Rotors wear out before they rust though normally and a little rust like that is far from unheard of.
Doesn't hurt to keep an eye on it though, & I wouldn't leave it sitting soaked in road salt if I could avoid it.
You may want get underneath the plastic and give the bike a once over too. If the frame is getting a bit of rust I would address that, if your exhaust is I'd hit that as well.
I'm not familiar with the foibles of the downtown but it's not uncommon for a vehicle to have a spot where the damp and salt like to sit.
If you're aware of it and check it now and again, maybe clean it and give it a spot of paint no big.
Normally no big, but it can, in extremis, weaken the frame at a weld, or a bare spot potentially.