Greeneochloa expansa is a rare grass with leaves that are simple and alternate, with flat to involute and firm blades ranging from 15 to 20 centimeters in length and 1 to 3 centimeters in width. The smaller uppermost leaves end in a hardened, sharp apex. The upper surface of the leaves is rough and pleated, while the lower surface is smooth. The margins are thin, and the throat has sparse hair. The sheaths tightly overlap around the middle of the stem. They are minutely rough, with a membranous to paper-like ligule about 3.5 to 4 millimeters long, slightly hairy, and a triangular to rounded, jagged apex. Its flowers are arranged in dense, oblong, paniculate inflorescences 15 to 30 centimeters long, with ascending branches bare of spikelets on the lower half. Long, silky, whitish-yellow hairs cover the tiny rachilla. The glumes are pale, equal in length, and persist beyond the floret, with a slightly recurved, pointed apex. The lemma is thin and somewhat papery, with a thickened and densely hairy callus, while the awn ranges from 7 to 10 millimeters. The palea is short and scabrous. The fruit is an ovoid, pale brown caryopsis, about 2 to 2.5 millimeters long, with a grooved ventral surface and a pointed apex. It inhabits wet forests, open bogs, and the edges of bogs between 950-2,290 meters.