Mycena mustea is a species of mushroom in the family Mycenaceae. First described as a new species in 2007, the fungus is known only from Kanagawa, Japan, where it grows on dead fallen twigs in lowland forests. The mushroom's dull violet to grayish-violet cap, initially covered with a fine whitish powder, becomes smooth as it matures, and eventually reaches a diameter of up to . The stem is slender, up to long, and is covered with stiff white hairs at the base. Underneath the cap are distantly spaced pale brownish gills that are narrowly attached to the stem. Microscopic characteristics of the mushroom include the weakly amyloid spores (turning bluish to black when stained with Melzer's reagent), the club-shaped cheilocystidia (cystidia on the gill edge) featuring one or more short knob-like protuberances, the absence of pleurocystidia (cystidia on the gill face), the diverticulate cap cuticle hyphae, and the absence of clamp connections. Taxonomy, naming, and classification The species was first collected in Japan by Hiraku Takahashi in 1999, and reported as a new species in a 2007 publication. The specific epithet is the Latin word mustea, meaning "fresh". The Japanese name is Sumire-ashinagatake. The infrageneric classification of the fungus is unclear, and depends on what taxonomic characters are deemed most important. According to Takahashi, the mushroom's violet pigment, the inamyloid (not staining when treated with Melzer's reagent) hymenophoral tissue (hymenium-bearing tissue), and the smooth hyphae of the outer layer of stem suggest a placement in the section Adonideae (Fr.) Quel., as defined by the Dutch Mycena specialist Maas Geesteranus. However, if greater taxonomic emphasis is placed on the weakly amyloid basidiospores, it would be more appropriate in the section Fragilipedes (Fr.) Quél. Description The cap is conical to convex to bell-shaped, occasionally with a low and broad umbo, and reaches in diameter. When moist, it is partly translucent, and grooves corresponding to the position of the gills under the cap can be seen. The surface is somewhat hygrophanous—it changes color as it loses or absorbs moisture. The surface is initially pruinose—covered with what appears to be a fine white powder (remnants of the universal veil that covered the immature fruit body)—but this soon sloughs off, leaving it smooth. The cap surface is a dull violet color when young, then becomes somewhat paler near the margin. The flesh is up to 0.5 mm thick, white, and lacks any distinctive odor and