On fallen twigs or moss-covered trunks of deciduous trees, or on fallen leaves of e. g. Salix and stems of different herbs, such as Filipendula ulmaria. Also found on fallen hazel nuts and small woody items in the litter. Early summer to early winter. In milder parts of the region all year. A common and widespread species. Fairly common in southern part of Norway but not recorded in the northernmost counties. Possibly overlooked.

Pileus 2-5 mm across, hemispherical, conical, becoming more or less parabolical with age, translucent-striate, shallowly sulcate, white-furfuraceous to floccose, glabrescent, white or grey. Lamellae 7-13, not always reaching the stipe, ascending, narrowly adnate or attached to a pseudocollarium, the edge convex, white. Stipe 5-30 mm long, filiform, straight to flexuous, equal, greyish-hyaline, puberulous, glabrescent with age, but often hirsute below, the base somewhat bulbous with a small, hirsute, white, basal disc. Odour none. Taste recorded as mild.

Basidia 14-18 x 7-9 µm, clavate or obpyriform, 2- spored, rarely 4-spored. Spores (2-spored) 8-10.5 x 4.5-6 µm, Q 1.5-1.9, Qav 1.6-1.7, (4-spored) (7-)8.2-8.8 x (3.8)4.5-5.5 µm, Qav ≈ 1.7, pip-shaped, smooth, amyloid. Cheilocystidia 11-32 x 7-18 µm, clamped, clavate or obpyriform to fusiform, mostly with a slender, straight to curved, simple or occasionally furcate neck up to 20 µm long; smooth or covered with warts or cylindrical excrescences 0.5-1.5 x 0.5 µm. Lamellar trama dextrinoid. Hyphae of the pileipellis smooth to verrucose, with terminal cells globose to obpyriform, densely covered with warts. Caulocystidia 20-110 x 6-7 µm, lageniform to cylindrical, smooth. Basal disc cystidia similar to the caulocystidia but typically shorter, often in chains of 2-4 cells, terminal cells 22-60 x 5-12 µm, cylindrical, clavate, lageniform, obtuse to subacute, smooth or rarely with a few coarse, apical excrescences. Clamps abundant in 2-spored form, very rare to apparently absent in 4-spored form..

Mycena tenerrima occasionally grows on hazel nuts. Two other species have been reported growing on hazel nuts: M. discopus (Lév.) Quél. and M. nucicola. M. discopus is a somewhat dubious species and awaits further description. Desjardin (1995: 79) excluded it as a nomen dubium. Huijsman (1958) described the cheilocystidia of M. nucicola as clavate, lacking an apical rostrum and Maas Geesteranus (1991b) did not report otherwise. Desjardin (1995) and Robich (2003) showed, however, that many of the cheilocystidia near the pileus margin have a single apical projection. A Norwegian collection showed long, flexuous rostrae as in M. tenerrima. M. nucicola can, according to Desjardin (1995) be separated from M. tenerrima on account of the 4-spored basidia, somewhat narrower spores and presence of acanthocysts on the basal disc.

M. tenerrima is supposed to have the following differentiating features: 1. 2-spored basidia 2. clamp connections common in all tissues 3. spores (4.8-)5-6(-6.4) µm broad 4. caulocystidia lanceolate, smooth 5. basal disc cystidia variously shaped, shorter than the caulocystidia, smooth or rarely with a few apical excrescences, often in chains. M. nucicola is supposed to have the following features: 1. 4-spored basidia 2. clamp connections rare on tramal hyphae (absent elsewhere?) 3. spores 4.2-5 µm broad 4. caulocystidia lanceolate with a broadened base, smooth 5. basal disc cystidia of two kinds: a) acanthocysts and b) elements similar to the caulocystidia, not in chains

The differences are rather marginal, and recent ITS sequences of collections determined as M. tenerrima and M. nucicola from Norway and Sweden indicated that the two taxa belong to the same species (Aronsen & Larsson 2015). Hence, M. nucicola should be regarded as a synonym.

A nitrous smell was noted on several Danish collections on hazel nuts that otherwise matched 4-spored M. tenerrima. Another 4-spored Danish collection on hazel nuts had ochraceous granules on the cap (Aronsen & Læssøe 2016). Jagers et al. (2022) found two distinctly different phylogenetic species of M. tenerrima in material from the Netherlands.

One problem is connected to M. adscendens var. carpophila (J.E. Lange) Desjardin, which seems to be very unsufficiently known. The type does not exist. Lange reported the species as having 4-spored basidia, narrower basidiospores, and fruits on Fagus pericarps, but he did not report on cheilocystidia shape nor caulocystidia, and he certainly was not aware of microscopic features of the basal disc. Until new material can be found it is impossible to tell anything certain about this variety.

Van den Berg et al. (2000) described the new species Mycena cecidiophila A.P. Berg, Berg-Block, Noordel. & Uljé, that was found growing on old knopper galls on the cups of Quercus robur. It differs from Mycena tenerrima in having a consistently brownish centre of the pileus and a conspicuously fimbriate margin. In addition it was characterized by the absence of pleuro- and cheilocystidia and a negative Melzer-reaction in the lamellar trama. The authors also proposed the new section Cecidiophilae to accomodate the new species. Later, this taxon has been shown to be identical with M. rhenana Maas Geest & Winterh. (Noten & Vannieuwerburgh 2009).

Both M. clavularis and M. corynephora may resemble M. tenerrima. Both have globose spores.