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, while visiting Relva VII in order to take the Starfleet Academy entrance exam, Wesley Crusher bumped into a Zaldan named Rondon 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. This incident was part of a test to gauge Crusher's ability to interact with other cultures and species. (TNG: "Coming of Age")