The vSphere interface reports that virtual machine consolidation is needed

When NetBackup begins a virtual machine backup, it requests a VMware snapshot of the virtual machine in vSphere. If the NetBackup policy is enabled for virtual machine quiesce (the default), the VMware snapshot operation initiates a quiesce of the virtual machine. If snapshot quiesce fails, the NetBackup job fails with status 156 and VMware snapshot delta files may be left behind in vSphere. Note: As a result of leftover snapshot delta files, the vSphere status for the virtual machine may warn that virtual machine consolidation is needed.

VMware has acknowledged the problem of leftover delta files after a snapshot quiesce failure. VMware has fixed this issue in certain ESXi versions. See the following VMware article for more information on this issue:

Delta disk files (REDO logs) are left uncommitted after a failed quiesced snapshot operation (2045116)

For a workaround for leftover delta files, see the following VMware article:

Committing snapshots when there are no snapshot entries in the Snapshot Manager (1002310)