Hi, which message specifically? Usually the message “visible only to administrator” is referred to notes or edit links added to the Newsletter plugic message, to make it easier for an administrator to edit the specific message or to understand the reason for that message (for example when an error message is shown).
When I had it set to Newsletter Public Page, I got that message when I was logged in. After much searching it was basically only an issue if you were logged in as an Admin.