Dohyun is being an ass, like staring at her snail form just to remind himself that she is not human is really getting old and not at all funny.
He is just using her for convenience, that's fine because she also almost forced him to accept their deal. But if he isn't going to treat her better or isn't even welcoming the thoughts of falling for her, they should just live for the convenience and let her be free outside of their agreement. He should tell her straight that he wouldn't fall for her so she can live the human life she's always dreamed of, and maybe fall for another person.
He is now getting jealous and "protective" when he cannot even accept her true form.
I only have had a second ML syndrome once. And this is the second time. Boss has so much self-awareness and so realistic. I'd bet if he ever finds out about her being a snail, he would be more accepting.