macOS does not play well with Linux so it is not unusual for the macOS Startup Disk System Preference not to recognize a non-Apple OS boot disk.
I've never used CentOS so I'm not sure how it behaves on a Mac, but it would not be my choice to use on a Mac since Macs do require special configuration of drivers that may not be included in CentOS by default. Also I have seen a Mac Mini have boot issues which was due to the video driver and/or video configuration in the Linux installer. I remember I had to use a special boot parameter to the Linux kernel in order to get some Linux disks to boot on a Mini. I don't recall the specific setting as that was a few years ago.
If you want to use Linux on a Mac, then I suggest using Linux Mint or one of the Ubuntu flavors such as Kubuntu or Ubuntu-MATE as they should just work on a Mac. I know a co-worker has been installing Linux Mint on our organization's old Apple laptops for years because it just works.
I know when booting Knoppix Linux on a Mac that the Mac appears to be frozen on the boot picker menu and/or a black screen until Knoppix reaches the Desktop. This is due to the Apple hardware/firmware behavior which is very poor. Make sure to give Linux lots of time to finish booting just in case this is happening to CentOS as well. My guess is you will need to provide a custom kernel parameter during boot.