The Marshmallow Mermaid: Princess Candy (Graphic Sparks)
Summary:  There's something fishy going on at Halo's school - the boy's swimming team members keep disappearing! When Halo goes to uncover what is going on, she finds herself head to head with one hungry mermaid. And she's not satisfied with just marshmal --Bookworming in the 21st Century BlogAfter an elementary school has closed for the day, a dangerous monster comes out of the schools swimming pool looking for food. He eats marshmallows and goes back in the water. When Halo arrives at school, she accidentally slips on something gooey and hi --Children's Literature Comprehensive Database