From the Super Mario Wiki
If Captain Toad or Toadette loses a level often enough, a sprout will appear with an Invincibility Mushroom. Plucking it and collecting the mushroom will cause the Toad to glow, similar to a Star. However, the effect lasts until the end of the level, making it easier for them to deal with tough enemies, leaving any puzzles within the stage as the only remaining challenge(s), similar to the Invincibility Leaf.
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