Roella muscosa is a flat-growing, herbaceous, perennial plant. It forms dense mat-like patches up to 1 m across. The stems are slender and woody in the lower section of the plant, becoming more herbaceous and branching in the upper parts. The leaves are crowded, ovate, with a petiole-like base; the margins are toothed.

Flowers are regular, sessile, solitary at the tip of stems; bracts are leaf-like. The funnel-shaped, pale blue corolla is divided into five lobes, fused into a long tube, with the base of the tube often fading from pale blue to white. Likewise, the five ciliate calyx lobes are fused into a short tube. Stamens are five, free and inserted at the base of the corolla tube; filaments are triangular at the base, and form a dome over a nectiferous disc; anthers are linear and attached to the filament at its base. The 2-locular ovary is inferior and crowned by the nectiferous disc; the style is rod-like with 2 glands at the base of a 2-lobed stigma. In each locule are numerous ovules. Plants flower from late November to late February.

The fruit is a capsule.