Spotted Spurge (Euphorbia maculata), photographed near the intersection of Fair Avenue and Mission Street in Santa Cruz, CA.
plant with reddish or purplish stems. Its stems and leaves contain a sticky sap. Its leaves are about 3-8mm long.
Purple spots are visible on a few leaves, leaf shape looks most like E. maculata to me.
My best guess at the exact species. Growing in sand near the river.