Ningyo to Ouji to Usotsuki Akuma

Overview

It was then that the mermaid discovered his first love... Merol is a beautiful mermaid prince known for his enchanting singing voice. His best friend is Krato, a flapjack octopus who is unfairly labeled as a “devil” by the sea residents. One day, Merol rescues Will, a human prince cast out to sea, leading to a fateful encounter that changes everything. Despite Krato's warnings about the dangers of befriending humans, Merol’s heart refuses to listen, and he finds himself falling deeply in love with Will..? (Source: Takeshobo, translated)

Source: AniList