From the Super Mario Wiki, the Mario encyclopedia
Li'l Marchers appear from red trapdoors when Yoshi triggers an alarm, during which they will follow him continuously. They will damage Yoshi if they bump into him, but destroy themselves in the process. Yoshi cannot jump on them because of the spike on their heads, but can briefly push them back with a thrown egg. Li'l Marchers are temporarily defeated if they bump into each other. Yoshi can make all the Li'l Marchers on-screen walk away by pressing on a yellow switch to silence the alarm.
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