did you get mushroom before you had the quest? If so, either try dropping it outside his rooms and pick it up after he gave you the quest. Click on it and move it in your inventory => watch if your quest log responds (turns red) - read the entry: does the log know you have the mushroom?
Or try this => click and move your mushroom within the inventory - maybe this triggers. If quest log responds => turns red, it should work.
