I was able to clean it up while upgrading the Docker version.
Stopped Docker (sudo systemctl stop docker)
Uninstalled Docker (sudo yum remove docker-ce)
Nuked the directory (sudo rm -r /var/lib/docker/overlay2),
then re-created the directory (sudo mkdir /var/lib/docker/overlay2).
Then re-installed Docker (sudo yum install docker-ce)
And restarted the Daemon (sudo systemctl start docker)
You need to shut down everything before doing this, and you may lose containers/volumes/etc. I have no idea if you can just do this without uninstalling and re-installing.