0:00 appears in my message bar on a grey shadow
I got rid of mine by restarting my iPhone. 

To add more description to the problem, It looks as if the area where I enter text has a voice memo with a duration of 0:00 where the text field should be. If I want to send a text I can still get a keyboard and I can still type a message, but the message I type is obscured by the grey, voice memo "box" until I hit "send." The text sends but the voice memo box remains.

Go to settings.  go to messages.  Toggle "show subject field".  Go back out to the main screen.  Go to settings again.  Go to messages.  Toggle "show subject field" again.  Go back out to the main screen.  It should be fixed.

Furthermore, the voice memo shows up in the text field of all of my text conversations.