The Greenery is found on the 5th floor. Its blocking the path to The Poisonus Hallway and Skull room. To remove it you need to Free The Bird. "BIRD EATS GREENERY"
In Extra mode it's only blocking The Poisonus Hallway. The Greenery also has the Green Candle, if you try to take it you will notice you can't reach it but if you keep trying to reach it The Greenery with take you by the neck and behead you leading to a 'Game Over'. You still need to free the bird to get writh of the Greenery but you need the Green Candle to do that.