Hi there,
Yes there is a command to remove the NFS mount. Just run the following from the shell interface:
esxcli storage nfs remove -v <VolumeName>
Replace <VolumeName> with the one you want to dismount. Before doing this make sure there are no VMs running on this NFS store on the ESXi server you are running it from. Once completed then try mounting again from the vCenter console.
Oh and yes it could just be the browser session. You might like to either try another browser or empty the cache before attempting the above.
Let us know how it goes