You might have noticed vCenter alarms regarding CVM memory usage being high,  and when you look at Prism console for the CVM, you will see it reports 100% but no prism alert will has posed up.

This is a known visual artifact with passthrough VMs that could be bypassed by creating a customize alarm in vCetner for VM memory usage and apply it to all other VMs or to the folders containing the VMs. 

Disabling the builtin vCenter alarms in conjunction with enabling this customized alarm can by pass the alarm reported for VCM while continue reporting any VM high memory usage.

For better understanding of this issue and its by pass please review the KB 4465 in your nutanix portal.