So how do the King of Mars, his Galactic Princess and their newborn son, X Æ A-Xii, spend a Saturday night holed up in their Los Angeles pad? A little anime — lately it’s been “Death Note” and “Evangelion.” Lots of late-night debates about the potential and danger of artificial intelligence. And many audiobooks and podcasts, particularly ones about history. “Right now, we’re going back to Genghis Khan for like the third time, and the Mongols, I guess,” said the Galactic Princess, otherwise known as Grimes, otherwise known as Claire Boucher, otherwise known as “c,” her initial and the symbol for the speed of light. “You seem to be obsessed with that,” she teased her boyfriend about Genghis Khan. Grimes, the singer and artist, and Elon Musk, the rocket man and Tesla magnate, have an otherworldly romance. Which works out well since Mr. Musk wants to occupy Mars, in case malevolent robots or an engineered virus threaten Earth, and then die on Mars, just not on impact.