It's called Kochou no Yumeji by author Shima. It's a webtoon.
The girl with brown hair is named Kochou. It was given to her after the god/youkai saved her life. She never mentioned her real name just that she wanted to feel needed.
The God/youkai name is Shiro-Gane; he has long silver hair and wears a white kimono with a blue sash on the left shoulder. He inserted a butterfly dream into Kochou so she could live, it's forbidden because humans have family who will missed the person who becomes a youkai. He has two helpers named mejiro and uguisu, one is in a red kimono and the other is in a light green kimono. Shiro-gane sells dreams to Youkais but he keeps odd hours so that's why he doesn't get many costumes.
They live in a town with youkais and the town but on the sides of a hole where if you fall down the hole you won't come back. The bother and sister youkais are cranes named miyuki (sister) and Miyuki (brother). Miyuki doesn't really hate humans because she saw her sister's love for her human husband. The brother hates humans because he blames them for killing their sister. They make kimonos which is how the older sister dies. Her husband tried to sell her a kimono that she made and brought unwanted visitors that ended up killing her when they were threatening the husband.
The mermaid one is true, Kochou altered the dream to help the boy mermaid find peace with his past loved one. His mom, the queen mermaid, was angry with the village that tried to harm her son. She made it rain until the town was wiped out and people died including the prince's love.
I searched under fantasy and webtoon in Mangarock.