How do I delete macOS installer that appears in boot options?

I have a 2013 13" MacBook Air with an Intel i5 processor, 128GB SSD, and 4GB RAM. It is running macOS High Sierra 10.13.6. As you can see in the screenshot, the remaining storage according to "About this Mac" is 63.3 GB, but in Disk Utility, it says only 48.69 GB are available. I noticed a macOS installer showed up in the boot options, even though I do not recall downloading Mojave, which I'm assuming that's what it's for. I could not find the installer anywhere in my applications folder, downloads, or documents. The only way I knew it was there is because it appeared in the boot options. I have no idea how to delete it, and it's gobbling up over 10 GB of storage.