Sea Witch's Calling is a craft in the Mermaid workshop. It needs 10 purple coral, 5 cursed pearls, and 15 tideblooms. Takes 1 hr.
It won't show up until you get to the goal to clear a cursed reef. Chop the whole reef. I left 2 hits on my cursed reef and was confused because I needed the Sea Witch's Calling and couldn't find it anywhere.