Scattered on fallen, decaying needles of Picea Abies and Pinus sylvestris. Only reported from Finland.
Pileus 0.5-2 mm across, campanulate to convex, shallowly sulcate, somewhat lubricous, densely white-furfuraceous, very pale blue but turning white soon after having been collected. Odour not recorded. Lamellae up to 8 reaching the stipe, ascending, narrowly adnate, white, with convex, concolorous edge. Stipe 5-20(-30) x 0.1 mm, narrowly fistulose, fragile, appearing glabrous above, sparsely and minutely puberulous below, said to be "viscid", like the pileus pale blue but turning watery white soon after having been collected, springing from a very small, cushion-like, white basal disc.
Basidia 13.5-18 x 7-8 µm, clavate to obpyriform, 4-spored, with sterigmata 2.5-3.5 µm long. Spores 7.4-9 x 4.3-4.7 µm, pip-shaped, smooth, amyloid. Cheilocystidia 12.5-18 x 5.5-7 µm, occuring mixed with basidia, clavate, covered with comparatively few, cylindrical, straight excrescences 1-1.5 x <0.5 µm. Pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis c. 2.5 µm wide, smooth or sparsely covered with scattered short excrescences, the terminal cells 10-20 x 10-20 µm and globose at the centre of the pileus, 13.5-40 x 10-20 µm and clavate to subcylindrical nearer the margin of the pileus, long- to short-stalked, densely covered with cylindrical, straight excrescences 1-1.5 x <0.5 µm. Hyphae of the cortical layer of the stipe c. 2.5 µm wie, smooth, with no trace of gelatinization, the caulocystidia, mostly in the lower part of the stipe 50-135 x 10-15 x 2.5-4.5 µm, narrowly conical or sublageniform, thin-walled, easily collapsed. Clamp connections absent
The macroscopic description has been taken from Harmaja's type description and Maas Geesteranus (1991). The microscopic detailes are based on Maas Geesteranus's (1991) re-examination of the holotype. I have not seen the type.
M. occulta is a member of sect. Sacchariferae and is characterized by the very small size, pale blue colour of both pileus and stipe that is turning white soon after collection, white-furfuraceous pileus, a cushion-like basal disc, 4-spored basidia, sparsely spinulose cheilocystidia lacking a rostrum, large, narrowly conical or sublageniform, smooth caulocystidia, and lack of clamp connections.
According to Desjardin (1995) M. occulta is most closely allied with Mycena mostnyae Singer, another species that forms pale blue basidiomata, described from Chile.