Clearweed. Pilea pumila.
Urticaceae, the nettle family.

Plant: Erect plant to about 2' tall with translucent stems, opposite leaves, and branching clusters of tiny greenish flowers in the leaf axils. Does not have stinging hairs. Grows in wooded or shady areas, often in large colonies. Perennial.
Flowers: Tiny, green or greenish white, about 1/8" across, in branching, axillary clusters (panicles). Female flowers have three sepals and a pistil, male flowers four and four stamens.
Leaves: Opposite, ovate, 1"-5", long-petioled, with three prominent veins, a somewhat wrinkled surface and a narrow, tapering tip. Margin with coarse, rounded teeth (crenate).
Fruit: Small, seed-like
Blooming: July-September
Habitat: Woods, shady spots.

Comments: A native member of the nettle family, this species is similar to the common stinging nettle, but lacks the stinging hairs. The common name refers to the translucent, watery-looking stems.

Where to find it: I have found one patch along the upper woods trail near the meadow transition.