"Magical crests" are often circular and filled with stars, words or numbers.
Hand of the Mysteries - Alchemy symbol of transformation
babylonsidhe: The Hand of the Mysteries goes by many other names, including the Hand of the Master Mason, Hand of the Philosopher, and the Emblematic Hand of Mysteries. (via Hand of the Mysteries - Alchemy symbol of transformation)
A previous pinner said, "Alchemical pelican. Although not mermaid/ siren, I like that she is emerging from a crescent moon, that looks similar two tails.