M. haleakalae grows in shrubs or small trees of tall, with new growth being glabrous or with scattered hairs. Its leaves are whorled and simple, with 4 leaves per node. Its blades are obovate to spatulate, with maximum dimensions of in length and wide. The petioles are 10–15 mm long, with no stipules. Its flowers are characterized as glabrous, fasciculate cymes below the leaves, with peduncles 0–1 mm long, and pedicels 1–3.5 mm long. Fruit capsules are of 12–13 mm wide, with 1–2 seeds per carpel, measuring 4–4.5 mm long, being glossy black when ripe, and shaped as crustaceous and ovoid, but sometimes angled due to compression.
M. haleakalae is endemic to the island of
Maui in Hawaii. == Phylogeny ==