The Ghost Fish is an enemy that can be encountered in Old Madryn in Momodora III.
They have two states: They are either nearly invisible and have no hitbox, thus being indestructible; or they shift onto the material plane, becoming normal enemies. They will only get aggresive if you come close enough to them. Also, be aware that they will shoot three projectiles upon materializing.
With the right timing, you can throw a paper seal into its hitbox, killing it instantly without any chance of it damaging you.