This folder stores guest OS index files used to search for and restore files within backed up VMs. So cleaning it up will not affect backups but will affect the ability to search in backup files. You can configure retention for index data in Enterprise Manager under Configuration > Settings > Guest File System Catalog
(please note, that Enterprise version of Veeam B&R allows for searching in both current and archived backups so make sure the retention is configured to keep only the required index data).
Btw, what Veeam B&R version do you use? Note, that v6 release reduced the size of the index in several times compared to the previous versions.
Also, this thread
might be useful.