Exactly TB. Tom is walking behind Rocco who is walking behind Michael. Thus, Tom is not even to the gate yet. Clemenza is sitting in the back seat of a hardtop on the street side of the vehicle and it's unlikely he has any view of Michael or Rocco, let alone Tom. Of course, the mall is occupied with Mafiosi, so what's the point of a signal.

As you state TB: Clemenza had his instructions.