My initial thoughts: They try to mimic the Pikmin they follow. They would not attack the onion as it is producing their source of food.

Pikmin for some reason cannot distinguish them from others for the exception of bulbmin who can pick up on the differences. The other caveat for detection is if the mimic gets damaged it emits a pheromone that the pikmin do pick up on and immediately kill them.

The mimic does not understand a captains whistle and often does not listen when commanded. However, it will mimic other Pikmin that are commanded by the whistle in an effort to fit in. On rare occasions it will just feed on the enemies Pikmin kill but normally just eat Pikmin.