Mei Lan, the Sea God.
Beloved deity of the Fishman.
The tales of the mermaid are indeed His divine artifact.
Yet it is known to all that not only Fishmen are His followers.
Faith spreads.
Any aquatic species might take up the faith in Mei Lan. Before He ascended to divine status, His true form was that of a mermaid. Hence, many from the mermaid clans also worship Him.
Similarly, it is not odd for humans to worship this deity, especially in coastal areas where His faith spreads more easily.
Historically, the Holy Bright Empire had no official Sea God.
And Mei Lan was quite the orthodox Sea God.
Believing in Him was very beneficial for His followers. It made sailing smoother and increased the fish catch. Sometimes fishing could yield large shells containing pearls or other rare fish species.
The key point was, He was a god of the other races.
The Holy Bright Empire had always fiercely oppressed such deities.