Flammulina

That is not Flammulina velutipes (Enokitake, or Velvet foot), because the wild specimen of these mushrooms have an orange cap with a darker center and a velvety stipe that most of the time starts off dark from the base, getting a lighter shade towards the cap. But indeed, the cultivated ones are white and tend to have a long stem and a small cap, but that's because they're being cultivated in a dark room/cellar/cave very close together. The length and slenderness is caused by the competition and the white colour is due to the lack of light, resulting in lack of pigments. And finally, the most important feature is that Flammulina velutipes grows on wood, for it is a wood decaying fungus. The picture is showing a Psathyrella, most probably, but even that is uncertain. Do you have better pictures of the underside of the cap and the stem?