From the Super Mario Wiki
Vine Slimes are strange slug creatures that crawl up and down vines in The Torrential Maze level in Yoshi's Story. These are most likely a distant cousin of the Slug. However, unlike the slug, these enemies are blue and can move their two eyes from one side of their body to the other. These foes are indestructible and should be avoided. They only seem to appear inside of the Torrential Maze area of Yoshi's Island.