Hi Enje... I would say your slug is Sakuraeolis gerberina or S. sakuracea. Are all three photos of the same individual? It is difficult to compare the length of the rhinophores in relation to the cerata in your images. One of the foremost experts of Korean opisthobranchs is Dr. Koh Dong-Bum, who resides in Jeju Island. He published a wonderful book, "Sea Slugs of Korea", a few years ago. If you have access to this book, please refer to page 193 for a positive ID on your slug. Please let us know what camera equipment you used. And thanks for sharing these beautiful photos. Nice show.