It is not a matter of "cheap" or not, it will happen to all zippers.
If they are cheap, they will break (the handle) when you try to open if stuck - the good/strong ones will hold (better at least)
Especially boardbags - here you often have two zippers, and maybe only use one of them.
The "left alone" one at the other end, will then get stuck without you knowing
And bags with tools (fins, spare footstraps, lines, epoxy etc etc) will be REALLY in the risk group for getting stuck, as you most often dont use these that often.
On the other hand, if you dont wear these bags to the beach normally, only stored in the car, they will last eventhough they COULD get stuck.
To get a stuck zipper free, I just pour a slant of hot water from the coffee boiler over the zipper, and that does it usually
In "tough" cases, you have to take a hammer and a piece of wood and hammer on the end of the zipper through the wood - this way you can get it free without destroying the zipper handle nor the zipper itself.
I think all bags and travelbags and wind-suits with zippers etc - "die" this way, only a matter of time (years) till they can be put in the garbage bin
So would be better to be able to remove the "white" from the area - as even when you get the zipper free, there is still some remains on this very spot and it might feel like a "bump" here.
I've never heard anyone finding something that work well actually, so really interesting now :
Contact cleaner ?
I must say I am a bit doubtfull about this... But would LOVE if there was a solution
PS: The O.P. is not talking about our usual well known "salt" issue it seems, but "rust" ?
Have never experienced this myself though.....