Calus only has one thing in this system that he cares about: the Guardians. Calus wants every Guardian in the solar system to hop into the Leviathan and help him retake his empire. He doesn't give 2 sh**'s about anything else. He's probably looking at the Dreaming City and wondering "How have those idiots not ended the curse already?". He doesn't even view us, the people he wants to recruit, as a threat, as he casually stated in the Spire of Stars raid armor that the Cabal have beaten a light-empowered race before, one that probably knew what was going on unlike us. His knowledge of the Universe is likely only surpassed by the Traveler and Darkness. He thinks the Worms, the Fallen, and the Nine are idiots doing meaningless crap. To him, the only thing worth fearing is the Darkness, as he knows that it can't be stopped. If Calus just came out and said he was working with Mara or vice versa, that would make no sense. Mara wants to save the system, Calus wants us to abandon/temporarily leave it.