Yes, spotted spurge (Chamaesyce maculata or Euphorbia maculata) is native to North America and can be found throughout the United States, including Washington. It's a common broadleaf weed that can grow in many places, including gardens, lawns, and fields.
Spotted sandmat is a member of the Euphorbiaceae (Spurge) family. Like most members of the family, it has a milky sap (latex). The sap of many spurge family plants is caustic or a skin/eye irritant. The sap of spotted sandmat is an irritant for many people although it has traditional medicinal uses.