I don't see where the law covers a kick to the face. You're allowed to raise a knee in a marking contest in order to protect yourself but Greene was actively kicking away in an attempt to push Dahlhaus off, and copped him high. Don't be surprised if it comes in as higher than low impact; it was a fair whack and there's a reasonable probability of an underlying fracture.
It will be explained away as careless contact to the head, but as others have identified, it happens plenty from behind with a speccy, it would have also been seen as fine if it hit him in the chest.

