Carpetweed. Mollugo verticillata.
Molluginaceae, the carpetweed family.

Plant: Prostrate, mat-forming plant, growing to about 12" across, with irregular whorls of semi-succulent leaves and umbel-like clusters of a few small, whitish flowers. Entire plant a bit unusual-looking for eastern North America. Annual.
Flowers: White to greenish, 1/8"-1/4", with five petal-like sepals and a protruding central structure. Flowers in umbel-like cymose clusters with 2-5 opening at once, arising from the joints along with the whorled leaves. Pink to sometimes white, striped, 1/2"-3/4", with five petals, two sepals, and five stamens with pink anthers. The flowers are organized in a loose cluster (a raceme) generally with 5-15 flowers.
Leaves: Narrowly obovate to spatulate, 1/2"-3/2", in irregular whorls of 3-8 different-sized leaves. Rather thick, semi-succulent, with smooth margins.
Fruit: An ovoid capsule
Blooming: June-October
Habitat: Sandy waste places, disturbed soil, roadsides.

Comments: Introduced from tropical America, often a weed, this plant is a member of a small, recently separated family Molluginacaeae that was previously included in the rather miscellaneous family Aizoaceae.

Where to find it: Uncommon in the park. I have found it near the eastern boundary with the medical campus.