HI MMASLOUH,
There is a threshold setup for the disk usage alert. It will be alerted in prism if the disk space exceeds the threshold limit.
Ex: Disk space usage for one or more disks on Controller VM 159.xx.xx.xx is 91% which exceeds the threshold of 75%
Please contact Nutanix Support to troubleshoot the issue.
For more info on thin provisioning, please refer to the following doc
Thin provisioning
Hope this helps,
Thanks