Your iMac was updated to High Sierra as part of a Genius Desk recovery from a lost/unknown firmware password. What you are seeing is the result of a
belt & suspenders[/b] precautionary technique to prevent loss of data in the process. Assuming your system is woking and you have not lost any data it is no longer needed. Since it shows up as [i]/Volumes/Backups of apple’s iMac it is either a partition or an APFS volume. In either case Disk Utility will reveal its specific nature* and is the only method available to get rid of it. If it is an APFS volume deleting it is almost completely risk free but removing partitions can be more dangerous. I would be
very careful to have a known good clone or other backup before removing it as accidents do happen.
FOOTNOTES:
* when you look at the drive in Disk Utility and "Backups of apple's iMac" is subordinate to a
Container then the drive is formatted APFS and you are dealing with an APFS volume otherwise it is a partition. to delete a partition see
these instructions. To Remove a volume follow
these