The second movie in the Tensei Shitara Slime Datta Ken series. 
After concluding the opening ceremony of the Demon Kingdom Federation Tempest, Rimuru and his companions are invited by the Celestial Emperor Hermesia of the great elven nation – the Magi Dynasty Salion – to visit her private resort island. As the group enjoys their brief vacation, a mysterious woman named Yura appears. A new incident unfolds against the backdrop of the boundless azure sea. 

(Source: Crunchyroll) 
Note: Relations are provisional and subject to change.

That Time I Got Reincarnated as a Slime the Movie: Tears of the Azure Sea