The conversation leading up to this exchange begins with Moneo discussing security considerations for the next day's parade. Leto is not very concerned with security however, and this increasingly worries Moneo. Remember, the parade ends up fatal for Leto. Leto is preparing a very unsafe event and waving off Moneo's efforts to make it safer.
Moneo believes he is concerned for the Leto's safety, and for very logical reasons. However, Leto can tell that subconsciously Moneo is actually afraid of the end of Leto's Peace.
The crystal sound of Moneo's voice rang in Leto's memories, speaking
things not cast in words. Moneo feared a universe where there was no
God Emperor. He would rather die than see such a universe.
Moneo is not aware he subconsciously feels this way. The rest of the conversation is Leto trying to make Moneo realize and overcome his fear, without directly addressing it.
This only makes Moneo more terrified. He keeps trying to cut through Leto's metaphors and get a direct answer out of him. Since Leto can see the future (or so Moneo believes), he wants Leto to cut the crap and just tell him what's going to happen tomorrow.
Instead, Leto calls him a liar. His "lie" is saying he's worried about Leto's suicidal planning when he's really worried about the downfall of the empire.
The "imperialism of consciousness" is being in charge of your own life. Moneo dreads a life without an omnipotent ruler to tell him what to do.
You can consider this in personal terms: Moneo's life really does consist of doing whatever Leto tells him to - it's his job. But Leto is also speaking to his role in galactic history: he's trying to free humanity from its recurring cycle of dependence and subservience.