I’m currently write a (long) series of articles about how to make your own Private Cloud using KVM/Proxmox and Ceph as backend. Ceph is a high available free software storage solution. It has 4 components: Managers, Meta Data Servers, Monitors, and OSD. The services are recognized by host, using the hostname of the servers.
It can happen that an extra server is appearing in the Proxmox web interface, but is not showed with the Ceph commands.
ceph -s ceph osd tree
In this case, the problem is due to a misconfiguration in the hostname of your server. In my case, you can see Ceph is configured with the short name but my server is using the full qualified domain name (FQDN).
To fix the problem, connect with SSH redefine the hostname from your server and restart the Proxmox node.
hostnamectl set-hostname node01 reboot
The ghost monitor will disappear from the web interface.