So my mail is showing 1 message, but it does not open?
Any ideas

I've got 39 of those and have for quite awhile now. My guess is these are very old messages that will eventually go away on their own if not opened in X amount of time? I'm sure I didn't open them for a reason (obvious spammer perhaps?) so I'm not concerned enough to go through months of messages trying to even find the messages but that's just me. Sorry I didn't have a better answer.

Are you using the app or the website? Have you blocked someone before opening the message in your inbox?