In this scene, [Kani] is reflected in the mirror, so it seems that the word [Nika] emerges when the character of the crab is reversed in the same way.
It seems like an unreasonable foreshadowing or a consideration, but in chapter 571 of One Piece, Navy Admiral Sengoku shows off his Devil Fruit for the first time.
Sengoku's ability is "Human-Human Fruit Phantom Beast Species (Model: Great Buddha)", so at least Nika wanted to tell him that it was a kind of Human-Human Fruit.