Slime

A Slime is a gooey creature made of slime appearing in Super Mario World 2: Yoshi's Island, its enhanced portYoshi's Island: Super Mario Advance 3, and Yoshi's New Island.

They mainly appear in Tap-Tap The Red Nose's Fort and King Bowser's Castle, carrying keys. They are disguised as rectangular platforms until a Yoshi touches them, which makes them transform. Like Salvo the Slime (who was actually a Slime Drop before Kamek's transformation), they cannot directly harm Yoshi and Baby Mario, but they push them. If a Slime is hit by a seed or an egg, they get smaller and smaller until they disappear. The same effect happens to the Slime if it falls into a Spike or lava pit. In Yoshi's New Island, one appears in Spray Day Mayday, in a room which can be reached via a door on top of a block resembling a disguised Slime.

Trivia

 * Nintendo Power's Super Mario World 2: Yoshi's Island guide considers this enemy to be Salvo the Slime. This is changed in the Yoshi's Island: Super Mario Advance 3 guide.