There is also an inner coupling that you can loosen up. There is an access hole with a set screw in it on the face of the turret. You will have to index each station to loosen each bolt.
I would suggest going to the Haas website and download the service manual for the Lathe.
Look up the alignment procedure and follow it. In addition to the turret coupling, there is a motor coupling may have slipped also which may need to be reset. Both procedures are covered in the manual but if you are not comfortable with either, contact your dealer to help out.