The revelation

With a few mistakes, it could easily lead to one's 'accidental' death and that would have made things easier for Jeremiah. Esme could have died during her transformation without him laying a hand on her.

But Esme did not show signs of transforming, giving rest to Jeremiah's doubts.

Yet, he let his men keeping an eye on her. And the incident when Esme shielded Aleister and saved him with the help of Ruby was reported to him. Ruby's identity was a mystery but it distressed Jeremiah and made him wary of Esme again. 

Soon, his people discovered pearls in the abandoned forest where Esme had passed from. One clue led to another until Jeremiah was certain that Esme's mer blood had not awakened yet. 

It was not until years later when her mer blood showed signs of awakening did he dispatch the Norwell Family to Visteria to capture her and bring her back.