Gregarious and very common on fallen, decaying leaves of Fagus. Also recorded on other types of leaves including Quercus and Salix (Aronsen & Læssøe 2016). Occuring late in the autumn. Common with Fagus in southern part of Norway.

Pileus 1-3 mm across, hemispherical, parabolical, conical to convex, often slightly depressed centrally, pruinose, glabrescent, sulcate, translucent-striate, whitish grey to pale grey or white, sometimes with darker centre. Lamellae 6-10 reaching the stipe, ascending, very narrow to fairly broad, the edge at first concave then convex, narrowly adnate to adnate, sometimes making a pseudocollarium, whitish grey to white, the edge white. Stipe 20-40 x 0.25-0.5 mm, filiform, flexuous, cartilaginous (?), pruinose, glabrescent except for the usually puberulous base, shiny, when very young entirely blackish or dark grey or blackish or dark grey at the apex, becoming grey to grey-brown, finally watery white, the base +/- somewhat bulbous, attached to the substratum by a patch of radiating, dark brown to blackish fibrils, not easily seen, and often seemingly insititious. Odour none. Taste not recorded.

Basidia 17-21 x 6-8.5 μm, clavate, 4-spored (rarely 2-spored). Spores 8.5-11.5(-13.5) x 2.8-4.5 μm, elongated, cylindrical (more cylindrical in 2-spored form), amyloid. Cheilocystidia 14-26 x 7-15 μm, forming a sterile band, clavate to obpyriform, covered with evenly spaced, cylindrical, straight, simple excrescences or warts 2-4.5 x 0.5-1 μm. Pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis 4-15 μm wide, densely covered with warts or short cylindrical excrescences. Hyphae of the cortical layer of the stipe 2.5-3.5 μm wide, densely covered with short cylindrical excrescences 1-2 μm long. Caulocystidia at the base of the stipe 5-15 μm wide, clavate, obpyriform or spheropedunculate, densely covered with short cylindrical excrescences. Clamps abunded in 4-spored form, absent in 2-spored form.

Mycena capillaris is a member of sect. Polyadelphia. It is characterized by the small size, the white to greyish pileus, the distant lamellae, the dark stipe (at least in younger specimens), the cheilocystidia and hyphae of the pileipellis covered with warts or short cylindrical excrescences, and the occurence on Fagus leaves. It is close to M. polyadelpha, but differs in several features as colour of pileus and stipe, the attachment of the lamellae, and the spore width. The latter is growing on Quercus leaves, but has also been recorded on Fagus leaves. Whether these records really represent M. polyadelpha, should be investigated by molecular methods. This applies also for records of M. capillaris on Quercus and Salix leaves.

According to Kühner (1938: 250) the base of the stipe is "naissant d'une petite tache brune ou noire, à peine nette à l'œl nu, formée de fibrilles noires rayonnantes, étroitement apprimées sur le support". Maas Geesteranus (1986: 163) described the stipe as "arising from a small whorl of radiating, very fine, dark brown mycelial hyphae (sometimes subepidermal and not easily discernible)". In my experience these fibrils are not easy to see, and very often they seem to be absent. Consequently this is quite useless as an identification character.