"In this direction." He leads Joker to the building, but once they're inside he lets Joker take over so he can take them to the correct apartment.

In the lift, Hythlodaeus leans against the wall and smiles at the other man. "I'm afraid I'm no good at giving nicknames, myself. I only ever thought to name one concept, and that was... well, a good few thousand years ago now. So if you'd like to spare us both the discomfort, what might I call you?"

Hopefully his persistence pays off. As for the mention of a concept- well, he realises that Joker won't know what he means yet. But he doesn't want to distract from his question, so he will elaborate afterwards.