Solitary, in small groups or subfasciculate on moss-covered trunks of deciduous trees (e.g. Acer, Fagus, Quercus). Common in the nemoral area, absent or rare in higher or more northern areas. In Norway rare, but rather frequent in Vestfold. Listed as NT in The Norwegian Red List (2021).

Pileus (3-)5-15 mm, hemispherical, conical, campanulate to convex, dry, sulcate, translucent-striate, pruinose, glabrescent, sometimes at first bluish to bluish grey becoming brown, darker in the centre, or dark blackish brown with bluish black centre, fading to pale brown with the centre somewhat darker. Lamellae 15-22 reaching the stipe, ascending, narrowly adnate, occasionally with a short decurrent tooth, white to grey-white or sometimes pale brown, the edge paler, occasionally with red spots. Stipe 10-65(-90) x 0.5-1.5 mm, equal, terete, hollow, straight to curved, pruinose, glabrescent, somewhat cartilaginous, at first greyish brown to brown with bluish apex, then grey-brown to dark brown, exuding a watery-whitish fluid when cut; the base covered with long, white fibrils. Flesh sometimes turning faintly red when cut. Odour indistinctive. Also experienced as farinaceous-rancid. Taste very bitter.

Basidia 22-28 x 8-11µm, clavate, 2-spored (rarely 4-spored), with plump sterigmata 7-10 µm long. Spores (7.8-)9.5-12 x 6.5-8.5(-9.0) µm, Q 1.3-1.7, Qav ≈ 1.5, broadly ellipsoid, smooth, amyloid. Cheilocystidia 20-66 x 8-19 µm, forming a sterile band, strikingly variable, fusiform, smooth, sometimes narrowed into a shorter or longer neck, or clavate, covered with few to numerous, coarse, simple to branched, straight to curved or flexuous excrescences 3.5-11 x 1.5-2.5 µm, or all kinds of intermediate shapes. Pleurocystidia numerous, fusiform, often longer than the cheilocystidia, generally filled with refractive globules. Lamellar trama dextrinoid. Hyphae of the pileipellis 1.5-3 µm wide, covered with more or less curved, simple to branched, cylindrical excrescences 3-10(-20) x 1-2 µm, which may form dense masses and tend to become somewhat gelatinized. Hyphae of the cortical layer of the stipe 1.5-3.5 µm wide, fairly sparsely covered with cylindrical, simple to occasionally branched, straight to curved, excrescences 1.5-13.5 x 1.5-2.5 µm, terminal cells infrequent, diverticulate. Clamp connections generally absent, but present in 4-spored form.

More microscopic figures.

Despite the many diagnostic characters, M. erubescens possibly often is overlooked and also at times misidentified. It can be confused with small specimens of M. polygramma and dark specimens of Phloeomana hiemalis. Neither of them has bitter taste nor is reddening in the stipe. Besides, the latter has smooth, fusiform to utriform cheilocystidia and inamyloid spores.

Mycena erubescens belongs to sect. Lactipedes, and is identified by: occurence on moss-covered trunks of deciduous trees bitter taste watery milky fluid in the stipe flesh more or less reddening when exposed to the air strikingly variable cheilocystidia fusiform pleurocystidia, often filled with refractive globules broadly pip-shaped spores