Why is there Messenger icon in App drawer?
No need to worry. As of Google+ v4.1, this is going away.


  5. Messenger is going away, Hangouts is sticking around
  Now that Hangouts provides messaging and video calls across Android, iOS and the web, we’re removing Messenger from the Google+ mobile apps (first on Android, then on iOS). Later today you'll be able to download all of your Messenger conversations (including text and photos) from Google Takeout.


The new app is rolling out now and you should have it in the next few days.