The reason Lex brought Z to the planet was because he was genuinely going to be very busy from now on. Between finding a cure or remedy to Moon's situation, meeting Belle, finding Liz, as well as coordinating the complete and total destruction of his grandfather's entire organization, he was not going to be available.
Not only did he want to ensure her security, but he didn't want Moon to be alone anymore. He was planning on slowly introducing her to people from the Inn so she got accustomed to them, so that eventually when she went to the Inn she wouldn't feel strange.
Now, however, he was beginning to doubt his plan.
Harry cleared his throat, attracting Lex's attention.
"It's really not my place to say this, but I think you should let them be. They're kids, and from what I can tell, it's been a long time since Moon had the opportunity to act like a kid, so…"
Lex's lip twitched.
