These creatures won’t automatically be exiled, though. Ichorid requires a cost to return to the battlefield, and both Bloodghast and Narcomoeba are “may” abilities. In this case, even if you want the trigger to go on the stack, it’s more strategically advantageous for your opponent to leave them in the graveyard, so they have the potential to be useful later.
Remember that Containment Priest also won’t stop Bridge from Below from generating Zombie tokens (assuming your opponent cast a creature and it died somehow), because the Priest cannot stop token creatures from being created, regardless of where they originated.
Today’s Rules Tip written by Jen Wong