Law 12 - VIOLENT CONDUCT
Violent conduct is when a player uses or attempts to use excessive force or brutality against an opponent when not challenging for the ball, or against a team-mate, team official, match official, spectator or any other person, regardless of whether contact is made.
In addition, a player who, when not challenging for the ball, deliberately strikes an opponent or any other person on the head or face with the hand or arm, is guilty of violent conduct unless the force used was negligible.
Porteous clearly and deliberately struck Alfie on the head with his upper arm/shoulder, when not challenging for the ball. The force was sufficient to knock Morelos down, so was not negligible and should have been a Red Card.
I noticed Tav having words with Porteous at the end of the game.