If MoonBin is salty about being left behind, he is saltier then the Dead Sea, after being abandoned in that brothel by literally everyone.
There wasn't much he could do alone, nor any place where he could go, as he still knows very little of the historical period they are in. Arguably, his historical knowledge is close to Boo's, but in his case it's because he lived in Nortrig for just few years, during which he focused more on learning everything else, especially the culture and habits of their own time.

Anyhow, salt aside, he still lives at the brothel and got a way to make a living there, without being a prostitute: he is their protector.
He can't say he likes to be there and to do that job, but at least he is eating and having a roof on his head. He actually can't really complain much, since he is treated well by everyone there, since they are grateful to him and they feel much safer with him around, even Lady Myu.

Many complicated things happened to him in the meantime, one of which the exact previous day, that is still keeping him terribly thoughtful.
Right now he is in Shiirhuang centre, wanding among the stands. He bought a few things already, but is intentioned to buy some more stuff before returning.

After QinShi's death, he turned pretty unwilling to do anything. He too has his limits and that was pretty much his. It was Jin and him to kidnap LianBi and thus causing the guards to come there, so he can only feel an overwhelming amount of regret. But there is still more to come.

It's dinner time for the guys from the future and he is not among them, but Jin and XianSeo are.
Suddenly, Haru receives a letter:
[ Noonim, I need your help. But I don't know where to find you, please help us.
- Huang MinHo. ]

At the exact same time, both Jin and XianSeo vanish, in front of everyone's eyes. Just poof.
The two don't even notice, it happens in a second and all that is left are their chopsticks falling on the table and their empty seats.

He is sat by the table of the kitchens, studying some potential jobs to earn money to start building their life in that era.
Suddenly he sees QinShi running by at the speed of light, but doesn't pay much attention to it, especially after hearing the door of the bathroom getting locked. "That must have been a real urgency", he mutters to himself.

Hours pass. He gets tired of his studies and starts to pack the stuff back, when he notices some fuss. Other servants went to call Lady Myu, whining how they "need to use the bathroom" and "it has been occupied for hours".
He slowly approaches them and the woman looks at him, seriously. "I have warned you, I was crystal clear about it! You are making yourselves way too comfortable!"
"Wait... what are you telling me...?" he gasps.
"A costumer claimed to be served by your short friend. I've tried to persuade him to choose among one of my experiences boys, but he was determined. I did all that was in my power, but he is a Sacred Blood and, as I have told you all before, I won't put myself or my people to risk."

"You are much mm ore knowledgeable and experienced than I am in the history of our world and even the recent times. I've arrived only that late and know almost nothing about anyone."
Jin is looking through the window, thoughtfully, with his arms wrapped around himself. "I know for sure that not all the Avatars were born such. I know that BangSul comes from Earth. I know that, when I was in my last years of school, BangSul was called Tero and seemed sick most of the time, deeply sick, physically and mentally exhausted. I also know that someone was fatally injured during an accident, when the professors organized a game for Zimkeshalhor at the school. That person survived, mysteriously, and lately turned out to be an Avatar too."
His eyes widen. "Are you sure? You think that someone has the ability to turn people into Avatars?" he gasps.
"I'm not sure. But I think that professor Yang was involved with both the mysterious situations."

When they realized to be back in the past, it's not that they just accepted it and gave up. They tried all they could in order to get back to their rightful time.
They have even tried to portal on Earth but the situation wasn't much better there: it's as if they are stuck in a past age and wherever they go, it's going to be the same.

Seen that there is no way back, Lay starts to make his mind on the situation.
For how much he would prefer to stick with his family, he knows that there is only one person among the others he can rely to, the only other 'adult' of the group: MoonBin.
He decides to take him aside and have a little conversation with him, to share his own thoughts and ideas.