Ghost Stream

Ghost Streams, also described as Ghost Riders , are wispy entities that appear in Yoshi's Story. They are long and slender and lack any bodily features. In-game, they come out of certain holes in the walls of Torrential Maze and quickly attach themselves to another hole, forming a platform with their bodies that Yoshis can climb. After a few seconds, they will enter the hole they connected to, and they will reappear shortly after. They are harmless creatures and will squish if a Yoshi steps on them.

Trivia

 * The official Shogakukan guide for Yoshi's Story says that Ghost Streams can be found in the areas that contain Vine Slimes in-game. The guide also makes no mention of the Vine Slimes or their vines at all, suggesting that they replaced some Ghost Streams. The holes Ghost Streams come out of can also be seen in-game where the vines are found.
 * When Ghost Streams appear, they emit a laugh normally characteristic of Boos.