The option to perform a cross-host Storage vMotion (Change both host and datastore) is grayed out in the vSphere Client, but the option is available from vSphere Web Client.
This issue occurs because the cross-host Storage vMotion functionality is not available from the vSphere Client.
You can use a cross-host Storage vMotion to migrate virtual machines to a different host and datastore simultaneously. In addition, unlike Storage vMotion, which requires a single host to have access to both the source and destination datastore, you can migrate virtual machines across storage accessibility boundaries.
Cross-host Storage vMotion does not require environments with shared storage. This is useful for performing cross-cluster migrations, when the target cluster machines might not have access to the source cluster's storage. Virtual machine processes continue to run during the migration with cross-host Storage vMotion.
As noted in the Cause section, the cross-host Storage vMotion functionality is only available when using the vSphere Web Client.