A Mellanoid slime worm was a somewhat despicable creature and, when calling another person by this name, it could be considered quite a provocative comment.
In 2364, Wesley Crusher bumped into a Zaldan named Rondon on Relva VII and, while trying to apologize, was called a Mellanoid slime worm among other insulting names. Realizing that the Zaldan was infuriated by politeness, Crusher stood up to Rondon and asked him if he wanted the confrontation to become violent. The Zaldan smiled, called Wesley his friend, and departed. (TNG: "Coming of Age")