Boo Blah

A Boo Blah is a kind of ghost that appears in Super Mario World 2: Yoshi's Island (and its reissue) and Yoshi's New Island. This creature sticks to the ceiling or the floor and stretches out periodically, similarly to Stretches from Super Mario Bros. 3. Boo Blahs are generally invincible, though they hurt the player only if they make contact with them while they are extended. Stomping them stuns them for a brief period of time, and by holding the jump button, a Yoshi can bounce off them. They can be defeated by using a POW Block or a cloud item. Also, there is an unused variant that has a Piro Dangle on its head, which was used in an earlier build of the game.

Colored Boo Blahs appear in both of the later versions of The Legend of Zelda: Link's Awakening in the optional Color Dungeon. For The Legend of Zelda: Link's Awakening DX, they are localized as Camo Goblins, and for the Nintendo Switch remake, they are localized as Goo Specters. They appear in red, green, and blue versions and disguise in their small forms against same-colored floors, pursuing Link before stretching out.

Super Mario World 2: Yoshi's Island

 * Shogakukan guide: 天井や床にはりついていて、ときどきからだをびよ～んとのばして驚かす. おばけはもう死んでいるので、倒せない. (The ghosts are on the ceiling or floor, and sometimes startle people by stretching out their bodies. Since they are already dead, they cannot be defeated.)
 * Player's Guide: Lurking in the stonework of Bigger Boo's castle, the Boo Blah's "Boo!" is worse than its bite.

Yoshi's Island: Super Mario Advance 3

 * Shogakukan guide: 天井や床にはりついていて、時々体をビヨーンとのばしておどろかす. 倒すことはできないが、ゆかにいるものをふんでもダメージは受けない. (They sometimes stretch out their bodies to scare you. They cannot be defeated, nor can they be harmed by stepping on them.)