It’s in line of fire of muck and water coming off the tyre. So it’s going to pick up a bit of surface rust. Some light grease can protect but otherwise just leave it.
As others have said, this is not worth losing sleep over. If your bolts are not stainless, they will all rust if stored wet. For instance, wash off the oily coating that new chains get treated with in the factory, which you should do if you ride in wet conditions since the stock lubricant won't cut it for long, then leave your chain wet overnight and by morning you can bet you will see rust developing.
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