Question: Strange EFI bootcamp issue
I have a bit of a strange issue here (this is going to need quite a bit of explanation, sorry).
For several months I have been booting Remix OS (a now-discontinued Android OS designed to emulate a desktop environment) on my iMac (late 2015) from a USB flash drive. I recently discovered that, despite having a 32GB flash drive, only 4GB are being recognized as storage space for RemixOS. I set out to increase the storage space; according to the walkthrough I used, doing so required installing Android x86 on the drive and creating/selecting a partition to boot from.
Unfortunately I misunderstood the instructions and did the wrong thing; when I tried doing what the instructions said, the installation said there was not enough space available, and rebooted. Now when I boot up the iMac and hold Option, an extra boot drive shows up, with the internal hard drive icon, named EFI Boot. If I select it, a terminal-like screen appears telling me to select the OS to boot into (both appear to be versions of RemixOS); selecting one will prompt a message saying that the boot path does not exist, and then the computer will re-boot into macOS.
I tried to install Android x86 on the USB again, but this time I apparently managed to do the same thing, this time creating a partition off of our external hard drive! Thankfully nothing appears to have been erased, but at this point I started worrying I might actually erase something and so I stopped.
When I go into Disk Utility, however, there doesn't appear to be anything extra or incorrect. So my question is: how do I get rid of these hidden drives without screwing anything else up?
iMac, macOS High Sierra (10.13.3)