The mushroom's dull violet to grayish-violet cap, initially covered with a fine whitish powder, becomes smooth as it matures, and eventually reaches a diameter of up to 10 mm (0.39 in).

Microscopic characteristics of the mushroom include the weakly amyloid spores (turning bluish to black when stained with Melzer's reagent), the club-shaped cheilocystidia (cystidia on the gill edge) featuring one or more short knob-like protuberances, the absence of pleurocystidia (cystidia on the gill face), the diverticulate cap cuticle hyphae, and the absence of clamp connections.

The surface is initially pruinose—covered with what appears to be a fine white powder (remnants of the universal veil that covered the immature fruit body)—but this soon sloughs off, leaving it smooth.

The cap cuticle is made of parallel, bent-over hyphae that are 2–6 μm wide, cylindrical, and covered with scattered, warty or finger-like hyaline (translucent) thin-walled diverticulae.

The layer of hyphae under the cap cuticle are parallel, hyaline or pale violet, dextrinoid (turning reddish to reddish-brown in Melzer's reagent), and contain short and inflated cells that are up to 25 μm wide.

[2] Mycena mustea is similar to the North American species M. umbrinovinosa, which is distinguished by having a vinaceous-brown to purplish-black cap, irregularly shaped cheilocystidia that are covered at their tips with long, flexuous excrescences, and clamp connections.

Mycena mustea is also similar to the European species M. urania, which differs in its blackish-violet cap, broadly club-shaped cheilocystidia covered with numerous, evenly spaced warts, and clamp connections.

Unlike that of M. fonticola, the cap of M. mustea typically becomes pale grayish-purple when mature; the cheilocystidia have several short finger-like excrescences at their tips; and the stem cuticle is made up of smooth hyphae.

In contrast, the cap of M. fonticola becomes dark violet-brown when mature; there are no excrescences on the cheilocystidia; and the hyphae of the stem cuticle are sparsely covered with diverticulae that resemble warts or fingers.