asier answer. The description sounds like him from the movie maleficent. So while I HOPE its triton (played by willaim Shatner:P) I think its Stefan.
Actually with Ursula coming to Once there is a good chance that is King Triton. Think about it how can you bring in Ursula without even considering bring King Triton especially his all powerful trident. Since she did desire to control the trident and become the new ruler of Atlantica. I won’t be surprised if it his him, since he is only character I know that fits the description. If it isn’t him then it is a new character, but like you I hope its him. Although I am not sure about Shatner, since I would like to see him be brought into a show with a character who is from out of this world like Dr. Jumba Jookiba (Lilo and Stitch) or sea captain like Captain Smollett (treasure island). Although if he did get hired I would still welcome him to show, and become very curious about his performance.