How can this possible be right? 75 GB of System storage on a completely fresh installation of Mojave on a new MacBook Air 2018?
The grey portion is System storage. And I use CleanMyMaxX from time to time. This machine is two weeks old!
“System” is a catch-all. On my machine the “System” total is 174 GB and comprises Applications (50%), System libraries (35%), actual macOS code (5%) and various related caches (10%). The number varies and sometimes is larger due to TimeMachine snapshots. I’d suggest a utility such as DaisyDisk will help you dig into the details if you want.
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okay thanks. Just seems completely strange to me because my old Air was not at that level for system storage at well. I will look at DaisyDisk, but how will that help if it is System storage?
I like OmniDiskSweeper for checking out how much space I’ve used.
There’s also a distinct possibility that it’s local Time Machine snapshots.
You can check via Terminal.app with this command:
tmutil listlocalsnapshots /
It will show you all of them. You can manually delete them too. So far the best tool I’ve found for deleting them (besides Terminal) is actually Carbon Copy Cloner which has a 30-day trial if you don’t have a license for it. CCC shows you the size of each snapshot, which I don’t know how to find via Terminal.
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I haven’t backed up this Air to Time Machine yet so I don’t understand how it has anything to do with TM 
DaisyDisk (or any of its competitors) will let you dig down into all directors to see what’s taking up space, and you can make decisions about whether any action is needed. About This Mac doesn’t give you that kind of information.
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Could it be APFS snapshots filling up the disk?
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You’re right. Between yesterday when I posted above and today, the snapshotting process increased my “System” report in About This Mac > Storage by a substantial amount. Good observation @JKoopmans
I agree with @Wolfie, snapshots will automatically be removed. It seems MacOS Mojave is doing some of the same file management as is done on iOS, where you also have quite unexplainable “system” items popping up from time to time.
It will take some getting used to, but I have got used to it on iOS so probably will also get used to it on macOS. It will be difficult when you’ve always managed your storage to have at least 25% free at all times 
Wasn’t exactly worried about snapshots, thanks. But, in my experience they will persist long (days, sometimes weeks) after multiple TimeMachine backups.
As strange as it might sound, not using Time Machine could very well be exactly what is causing the issue 
If Time Machine is not disabled, and has not run yet, you are more likely to have a build-up of APFS snapshots.
Back before APFS, Time Machine used to do something called “local backups” (IIRC) which could fill up quite a bit of space. I think those are roughly the same thing (conceptually) as APFS snapshots.
The idea what that the Mac would make these snapshots / local backups when the Time Machine drive was not available, then when the Time Machine drive later _did _become available, those local backups would be copied over to the Time Machine drive, and then automatically deleted from the main drive.
In a world where laptops are increasingly more and more prevalent and Time Machine drives are not always connected, it was a good idea…at least in theory.
Like almost all good-ideas-in-theory, sometimes the implementation has flaws, especially if you don’t ever intend to use Time Machine, or if the snapshots don’t get deleted for some reason.
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And by Crashplan, I meant Backblaze.
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