I have a similar situation with an unknown virtual mobility master showing up in the hierarchy.
Tried removing the configuration for the device with:
no configuration device 00:15:5d:c8:13:38
Received: Error: Device 00:15:5d:c8:13:38 cannot be removed. Not found
show switches, does not list it as a MD, master or standby.
I upgraded to 8.7.1 yesterday, but the unknown MM was also present in 8.7.0.
Any suggestions on how to get rid of it? Thanks!