Moomin wasn't sure what to make of this place yet.
He was nothing if not optimistic, of course! People were just friends he hadn't met yet, right? He wasn't suspicious of them as much as he was nervous about messing it up. They were so ... different. Moomin has never had a responsibility in his life, and the idea of having ones that he wasn't even prepared for was ... terrifying. He hadn't even been ready to be an adult before he accidentally left the valley.
I like to be alone sometimes, he'd told Snufkin once. Snufkin had smiled knowingly up at him, settling on the riverbank, and simply told him not for long, though. How did his best friend do it? Snufkin had done the 'traveling' thing for far longer than Moomin ever had. He'd even found his way back from more distant locations. Even now, Moomin still felt the weight of his months of near-solitude. He thinks he understands now why Snufkin had told him that he wasn't allowed to join him for his travels.
He feels jittery. Normally when he felt like that he'd go outside and look for adventure, but he was ... he'll admit it. He was scared he'd get lost again. He still hadn't quite figured out how to navigate the caves yet. So, Moomin settles for wandering the near-empty tunnels, feeling small and lonely. And, habitually, he sings to himself. Snufkin had made the song for him, a long time ago. It did wonders to make him feel less lonely and less like he missed Snufkin.  It wasn't quite comprehensible, sung in a low voice in a strange amalgamation of the languages he knew. Well -- less so English as much as the others, really.