Challenge
Virtual Machines are grayed out after vMotion. vDisk is in "not monitored" state.
Cause
It is a known issue with SCOM that virtual machines become grayed out after migration. The state of vDisk will be "not monitored".
Solution
There is no final resolution for this problem. There is a workaround, and what you can do is Restart the System Center Management Configuration service. In case of agressive DRS settings, a scheduled restart of the System Center Management Configuration service can be set.
More Information
Based on our testing results, this issue is more related to SCOM UI, meaning that the virtual machines are actually monitored.