As far as I'm concerned it was an accidental collision and should have been treated as one. It was as much caused by the angled run of the Saints player driving away from his try line than it was by Moon. A lot of people say intent doesn't matter over shoulder charges, but I think that is ridiculous.
Occasionally there will be accidental clashes of head between attackers and ball carriers. We see those as what they are, regretful accidents. We don't punish them as deliberate headbutts when they clearly aren't. They are just a regrettable part of a tough, full collision sport.
If he'd clearly gone in with his shoulder to the players head when he was 3rd man in the tackle I'd have wanted him to get 10 games minimum. Moon simply didn't do that. I think after an accidental collision he should get SOS at worst. I think the just verdict is to strip the red card from him and say the VR over reacted, but I have no faith in that happening.