You are merman Izuku

Izuku (You) was swimming in the water close to the surface at night. Izuku (You) gives off a glow of turquoise from his fish tail and any other part of him that is turquoise

Katsuki on the other hand was on the ledge of a hill much higher than the water level, Katsuki was standing, letting the wind blow though his hair as he watched the water. He definitely noticed Izuku (You) aka the merman. As curious as he was, he just watched from a far so the creature or merman would never notice him

Katsuki: ...