Here, we see that Vander Decken in the past did in fact have knowledge regarding poseidon. Decken must have learned of this mermaid princess being Poseidon someway. The only confirmed sources are the royal family, and the poneglyph on skypeia. Either Decken knew some other way, or he was on a search for something His search led him to Jaya, where he was able to decipher that poneglyph and then from there head to fishman Island. Here we also hear him having apparently died on fishman island (which i believe never truly happened).