Doukyo Suru Neneki

Yuuji Fujiwara had an unlucky life. As a child, he was blamed for a vase he did not break, and he was fired from his job for a colleague's mistake. At the convenience store where he works part-time, Takamiya, a younger girl, blames him for mistakes made by others. When he returns home in a depressed mood, he is greeted by a strange girl. When Yuuji is puzzled, the girl reveals her true identity. It turns out to be a mucus creature that has been living with him for the past three months. The creature, which speaks human language and can change its shape at will, is in love with Yuuji. Yuuji is not at all pleased with the mucus creature's affection. In order to attract Yuuji's attention, the mucus creature changes its shape into Takamiya, the co-worker at his part-time job, and seduces him.

(Source: MediaBank, translated)