The Id Monster attacking the crew of the United Planets Cruiser C-57D.  The creature is normally invisible, but its interaction with the ship’s force field allows the crew to see it for the first time.

If you look very closely you’ll notice that the monster has a goatee, just like Walter Pidgeon’s character Dr. Morbius.  Morbius’ enhanced unconscious mind is responsible for creating the monster.

Walt Disney Productions animator Joshua Meador was “loaned” to MGM specifically to create this scene.  That was unusual move back in the day when nearly everyone was contracted to a specific studio.