In 5.1 and 5.5 NFS (assuming you are attempting to use NFS ?) doesn't have authentication options, most people secure this by restricting access to the share by only allowing specific IP address rather than a user / password authentication.
NFS 4.1 does allow kerberos authentication, but thats only in esxi 6. and comes with some limitations.
Do you have other vmware hosts accessing data stores on the Net App array ? how is access controlled / given to those machines ?