1. go to /media. View should be something like picture attached. Look into folders listed - the twin, in this case "cdrom/cdrom0" can be ignored -, there must be one empty. Note its name. Here it is "18e2ee88..."
2. go to /etc/openmediavault and edit config.xml (F4). Search for "<fstab>" and comment out the corresponding <mntent> section (<fsname>!) by bracketing it with "<!--" and "-->" like this:
3. go to /etc and edit fstab. There will be one line with the same name. Comment it out by putting a "#" in front of the line:
4. restart OMV. Your ghost filesystem should be away now. At least mine was away, after I did these steps.
As I find this out being valid for OMV3, I'm not quite sure, whether it is valid for OMV2 as well. But as I still have an OMV2 at company to compare with, I can look for differences if needed.
Edit: OMV2 at company does not show any differences.
Hope it helps.