No, Powerpath (and navisphere also ) is not necessary to make the disks visible in the vsphere .Once done registration and allocation through storage groups in navisphere it should be visible in Vsphere after a rescan (NS120 is a celerra-NAS model and clariion model should be CX4-120 which is the backend storage in the NAS).
You can use either native multipathing for enabling multipathing on Vsphere.