What kind of rust is this and is there something I can do to fix the zipper?

This is simpler than you think.

The zipper pull (the oxidized part) is likely made of pressed zinc (not aluminum which would be too soft). The best thing to do is replace the slider. The rest of the zipper is fine as the teeth are made of plastic (likely nylon). The good news is that you (or any tailor shop) can do the slider replacement very quickly - it's just squeezed shut with pliers - and there are repair kits available online and in fabric stores.

A wet bag will ruin everything.

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You can visit any tailor shop and they can change this part very easy.

If you have WD40, you can use that to clean the rust.

Rust is oxidation of iron.  My guess is that the zipper pull is made of aluminum.  In any case, the moisture has oxidized the metal.  You can't fix oxidation like that because it's a corrosion of the metal and not just a buildup on top of it.  At the very least, the zipper pull has to be replaced.  I don't even know how to match a pull to a zipper, so if it were mine, I'd look at replacing the whole zipper.  

If it was left in a wet bag, you should check over the whole tent for mold and mildew.  If those are present and/or the rest of the zipper pulls look the same, you may want to replace the whole tent.

The zipper is most likely aluminum which means that the accumulation is probably a zipper wax of some sort. Work it open and then apply replacement zipper wax to both sizes. Move the head back and forth until it moves freely.