Hiding the content of the message while the phone is unlocked! How can I hide the message content in this "drop down menu -- I don't know what's its name". It is hidden when the phone is locked I want to hide it when the phone is unlocked as well.
You will have to completely block the notifications from showing. Or go with the app @Firelord advised on the comments.

From WhatsApp's Configuring your notifications:


  Navigate to your phone's Settings app >
  Apps &amp; notifications > WhatsApp > App notifications . On most phones, you will have the option to:
  
  
  Block all notifications if you wish for no notifications and not even the WhatsApp icon to appear on your phone's status bar.
  


The above advice can be applied to any app.