Boo Blah

A Boo Blah is a kind of ghost that appears in Super Mario World 2: Yoshi's Island, its remake Yoshi's Island: Super Mario Advance 3 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. They are generally invincible though they only hurt the player 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, 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.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: Lurking in the stonework of Bigger Boo's castle, the Boo Blah's "Boo!" is worse than its bite.