Get Virtual Machine list reside on VMware Datastore
Virtual machine reside on datastore and you do not care in which cluster they are
(Get-Datastore -Name 'datastore_name').Extensiondata.Vm|%{(Get-View -Id $_.toString()).name}
To find Virtual machines on specific Datastore and specific Cluster
Get-Cluster "your_cluster"|Get-vm |?{($_.extensiondata.config.datastoreurl|%{$_.name})-contains "datastore_name"}
or
Get-Cluster "your_cluster"|Get-vm |?{($_.extensiondata.config.datastoreurl|?{$_.name -eq "datastore_name"})}