Complex sentence: Clara realises that as Cass refused to allow Lunn into the ship, he has not seem the writing on the wall, and therefore the message is not encoded in his brain meaning the ghosts won't attack him.
Split into simpler sentences: Clara realises that, as Cass refused to allow Lunn into the ship, he never saw the writing on the wall. Therefore, the message is not encoded in his brain, and the ghosts won't attack him.