Maleficent's eyes raked over the gestures Ariel was making, it all coming together in her head. "A mermaid, then." She nodded. So this was beginning to make sense. She had wanted to walk, and so she had found some tricky hag with which to barter.
"The first thing that needs to happen is to get you your voice back." Maleficent mused. "I'm finding it excruciating trying to hold a conversation with you."
Her tone suggested irritation and impatience, in reality, Maleficent wanted to undo this curse. To give this girl a second chance. To show her that the world could be hers, even without love.
She didn't wait for the mermaid to agree with anything. The green light began to emanate and the magic flow out of the fairy as she lifted her hands. "Redde quod capta sit loquatur." The spell was nothing rehearsed, the words perhaps not even necessary given her skill, but some, she'd found, appreciated hearing. So Maleficent told the magic to return what was taken, to give Ariel her power of speech back.
Whatever needed to happen, whatever needed to be replaced, would.