When a slave server monitored by mariadbmon replicates from a server that the monitor cannot see, it gets assigned the Slave of External Server label. If the server doesn't have read-only enabled, it will be assigned the Master, Slave of External Server, Running status and all other servers of the same type (slaves replicating from a master outside of the monitor configuration) will be assigned the Slave of External Server, Running status.

To make it possible to use mariadbmon to monitor slave servers replicating off of a Galera cluster, the slave servers that are replicating from an external master and have read-only enabled would have to be treated as slaves.