Not configuring a cleaner schedule could also impact the cleaner full cycle completion
So current functionality The cleaner will auto start during system idle time. Once a backup starts, the cleaning process will stop, not completing it's work. So if there is no full cycle completed it could add to pending work. Pending work mounts and that is part of the reason for the message.
Recommended actions:
Set a cleaner schedule to allow the cleaner to run full cycles.
The cleaner auto start when the system is idle process will be more effective as well since there will be less to clean
For immediate impact, if possible consider suspending backups and manually running the cleaner to clear current pending work. Based on the amount of pending work that process could take some time to complete, keep that in mind if suspending backups.
However, the schedule without the manually run should eventually catch up.
I'll include cleaner best practice guide. https://support.quest.com/dr-series/kb/228226/cleaner-best-practice