What is leech essence in snail girl?

In this story, leech essence is a villain.

In China's folk story "River snail girl", leech essence likes River snail girl very much, but River snail girl doesn't like him. So leech essence decided to take snail girl away. He pretended to be a fortune teller and cheated the snail shell from the blind mother-in-law of the snail girl.

With the snail shell, the snail girl was caught by leeches and couldn't get out. Farmers and their partners went to the leech cave to save the snail girl, but they were listless by leech. Later, they came up with a way to sprinkle salt on leech essence. What leeches fear most is salt, and finally they die painfully.