Mojave on Mac OS Extended? I am running a 2012 MacMini, and recently installed an SSD to replace the spinning HD it came with. 
After partitioning to 2, I CarbonCopyCloned the Mojave OS to one partition, and an installation of Yosemite on the other. 

Now I notice that the Mojave drive is MacOS Extended, not APFS. 
The question - can I leave it like this, or will i run into issues? If this situation is bad, I can boot Mojave from an external drive and reformat that partition. Just asking if this is necessary.
I would suggest putting on APFS as you have SSD drive even the current situation will not hamper anything but you can use the optimal resource as required when you put on APFS. 


APFS, or Apple File System, is the default file system for solid-state drives and flash memory in 2017’s macOS High Sierra.
If you’ve got a mechanical hard drive, and you intend to use it only with Macs, it’s probably best to stick with Mac OS Extended. And any drive that needs to work with older Macs, running El Capitan or earlier, should absolutely be formatted with Mac OS Extended because APFS is not compatible with those computers.