It works with the patch, but what are the controls? I can't start a new file no matter what button I press, although I can still use the directional buttons
That power-up is a complete PITA. The yukkuri mushrooms are also frequently used for evil in some later stages. Yukkuri Marisa isn't easy to control (you can't easily stop, for starters) and the mushrooms will chase your ass, so naturally the developers hid them in some item blocks in areas where you wouldn't need or expect the yukkuri shroom.
On another note, yukkuris are also in The Genius of Sappheiros. They're basically that game's metal slimes. (Youmu also learns a skill called Slow Cut. "Cut it easy!")