From Rain of Snow Translations:
A weird "blue rain" started falling on An Mu's city, resulting in many people being unable to sleep well, after getting to know that a organization “Qing” will enter the dreams of people that have been unable to sleep well, and kill monsters, An Mu decided to join the organization and kill those monsters to level up and become strong, his only motive – is to save his crush’s dream

Original Webtoon