Single or in small groups on fallen, decaying leaves of Salix and Betula in mainly alpine sites. Autumn. Found in four different counties in Norway and in Finland. Rare or overlooked.

Pileus 0.5-2 mm across, covered with a separable, gelatinous pellicle, conical to broadly campanulate, parabolical to convex, sulcate, translucently striate, delicately pruinose, pale brown, sometimes becoming paler to greyish white with a somewhat darker center. Lamellae 5-10 reaching the stipe, ascending, adnate, white. Stipe 3-13 x 0.1-0.2 mm, pruinose, glabrescent, terete, straight to somewhat curved or flexuous, equal or somewhat widened below, hyaline greyish, springing from a pubescent, white, 0.5-1 mm wide, basal disc. Odour indistinct. Taste not recorded.

Basidia 18-23 x 9-13 μm, broadly clavate to obpyriform, 4-spored, with sterigmata 2-10 μm long. Spores 7.5-11 x 4-6 μm, Q 1.4-2.3, Qav ≈ 1.5-1.9, pip-shaped to subcylindrical, smooth, strongly amyloid. Cheilocystidia 12-25 x 7-14 μm, forming a sterile band, very variable in shape, clavate, subfusiform to obpyriform or more irregularly shaped, with fairly few, simple to branched, cylindrical to clavate, curved to flexuous excrescences 1-45 x 0.5-4 μm. Pleurocystidia not seen. Lamellar trama dextrinoid. Hyphae of the pileipellis 1.5-4 μm wide, embedded in a gelatinous matter, smooth or occasionally with one or two short excrescences, the outer surface of the pileipellis concisting of coarser, diverticulate hyphae 2-7 μm wide, covered with warts and short cylindrical, simple to branched excrescences, occasionally some much longer, cylindrical, flexuous excrescences 1-30 x 0.5-2.5 μm, terminating in clavate, diverticulate, acanthocystoid cells 7-20 μm wide. Hyphae of the cortical layer of the stipe 2-3 μm wide, smooth, caulocystidia scarce, 9-17 x 2-6 μm, subfusiform, clavate, subcylindrical to rather irregularly shaped, covered with very coarse outgrowths. Clamp connections very rare, seeming almost absent(very few seen at the base of the basidia and the cheilocystidia, and at the septa in the pileipellis).

Examined material:
NORWAY: SOGN OG FJORDANE, Gaular, Kleppe, Furuhaugane, UTM 32VLP 1682 0910. På nedfalne bjørkeblad i bjørk-furusumpskog. 240 m.o.h. 27.09.2012, Leg. Arne Aronsen, Inger Kristoffersen, Edith Ramberg, Berit Nyrud og Ulla-Britt Bøe. Førde, Rørvikfjellet, UTM 32VLP 5189 1087. På nedfalne bjørkeblad i fjellbjørkeskog med blåbær og bregner. 545 m.o.h. 28.09.2012. Leg. Arne Aronsen, Inger Kristoffersen, Edith Ramberg, Berit Nyrud og Ulla-Britt Bøe (HOLOTYPE O-F76006). Lærdal, Hemsedalsfjellet, UTM 32VMN 5046 6431, 1100 m.o.h. På nedfalne råtnende vierblad i alpint vierkratt. 30.09.2012. Leg. Arne Aronsen og Inger Kristoffersen. TELEMARK, Hjartdal, Gaustakneet, Stavsrobakken, UTM 32VMM 84119 32706. På nedfalne fjorårsblader av Salix i alpint vierkratt. 1190 m.o.h. 22.09.2012. Leg. Arne Aronsen; Tinn, Gaustakneet, Langefonn, UTM 32VMM 8247 35557, 1060 m.o.h. På nedfalne fjorårsblader av Salix i alpint vierkratt. 22.09.2012. Leg. Arne Aronsen; Tinn, Hagestaul, UTM 32VMM 82346 36471. 983 m.o.h. På bjørkeblad under bjørk, vier, bregner. 04.10.2014. Leg. Arne Aronsen; Tinn, Haugestaul, UTM 32VMM 82346 36471. On fallen, decaying leaves of Betula in habitat dominated by Betula, Salix and different ferns. Alt. 983 m. 05.10.2014. Leg. A. Aronsen. Vinje, Strandbekk, UTM 32VMM 52839 23011. På nedfalne bjørkeblad i kvisthaug. 14.09.2014. Leg. Arne Aronsen. BUSKERUD, Hol, Ustaoset, UTM 32VMN 48550 06749. På nedfalne vierblad i vierkratt. Ca. 1000 m.o.h. 08.09.2012. Leg. Arne Aronsen. OPPLAND, Lunner, S. Oppdalen, Mørkomdalen, Hellerud NE, Southern/middle boreal, oligotr. (Vacc. myrt.) spruce forest (submesic-poor); planted after clearcutting 1956; on nerve of old leaf of Betula, in Sph. quinquefarium, UTM NM 9337 8497, 450 m.o.h., 03.10.2001. Leg E. Bendiksen EB 203/01 (O- F- 169163. TRØNDELAG, Steinkjer, Skrattåsen 18.09.2021, on Betula leaves. Leg. A. Aronsen.
FINLAND: Perä-Pohjanmaa Prov., Rovaniemi, Kiimamaa, Nature Protection Area, Grid: 3421701 7408399, On fallen leaves of Betula. 06.10.2013. Leg. Arne Aronsen, Tea von Bonsdorff and Anders Wollan.

In many aspects Mycena mucoroides resembles the well known species M. mucor, which is known to inhabit fallen, decaying oak leaves. It differs, however, in having a darker, more brownish pileus, a somewhat larger basal disc, broader spores, cheilocystidia with longer and coarser excrescences, a pileipellis with acanthocystoid terminal cells, and entirely different, much shorter, more irregularly shaped caulocystidia. Five collections were sequensed and analysed by Ellen Larsson, Gothenburg and compared to two sequences of M. mucor. The analysis showed that M. mucoroides and M. mucor are two separate species.

Mycena mucoroides seems to have a preference for alpine sites, and it has been collected up to 1200 m. in Norway, but it has also been found in similar habitats in lower areas.