From the Super Mario Wiki
They mainly appear in Tap-Tap The Red Nose's Fort and King Bowser's Castle, carrying keys. They begin in disguise as strange large blocks 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 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.
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