Cloud Drop

Cloud Drops, also referred to as Blusties or erroneously Gusties, are cloud monsters that fly in a short line vertically or horizontally in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, Yoshi Touch & Go, Yoshi's Island DS, and Yoshi's New Island. They appear in packs and in areas where Yoshi can turn into a Helicopter. The move quickly, but they can be defeated with a hit on the head.

In Yoshi's Woolly World, enemies resembling Cloud Drops, known as Wiks, appear.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: Watch out for these wherever the billowy white stuff is found on Yoshi's Island.

Yoshi Touch & Go
(It will move back and forth along the same route in the air, either up and down or to the left and right. It is a good idea to draw a cloud to block the route, stop them in their tracks, and then take them down. If there are many of them, it is safer to build a wall with clouds and confine them on the other side.)
 * Shogakukan guide: 空中の同じルートを、上下または左右に往復する. ルートをさえぎるように雲を描き、足止めしてから倒すといい. たくさんいるときは、雲で壁を作って、反対側に閉じこめてしまえば安全.