This ghost iron golem bug occurs when you use an iron ingot to heal a dying iron golem.
ghost iron golems are immune to all damage. Projectiles fired by hostile mobs pass through the golem's body, and attacking mobs that come close show an attacking motion, but do not take any damage. However, when the Ghost Iron Golem attacks a hostile/neutral mob, it will still attack.

A Warden is perfect for summoning a ghostly iron golem. Since the Iron Golem dies after taking 4 melee attacks from the Warden, it can be created by healing it using an iron ingot when it dies after being hit by the 4th melee attack from the Warden.

If you succeed in summoning a ghostly iron golem, the texture of the iron golem changes to look dead or not dead, the golem becomes invincible, and the warden continues to take damage and dies.

Meanwhile, if you move away from the ghost iron golem created through the Warden to the despawn distance and then approach it again, you will see the golem standing fine, but it still cannot be hit and takes no damage.

The only way to get rid of a ghost iron golem is to type the kill command.