‘nx’ user account is created only for NoMachine services, you are not able to login on this account using any other protocol than NX. That’s why this account is hidden.
If you need to make ‘nx’ account visible you can change an entry in registry:
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList\nx
change value from ‘0’ to ‘1’