What Ridley means is that the original alien, the one birthed from humans, won't be in the prequels. He wants to redesign a new alien cause he thinks that since we all know what an alien looks like, it has taken away some of the fear factor when the creature attacks (we all know what an alien looks like and what its traits are; so if we know this, why should we be afraid of something that we have faced in multiple movies and video games?).