In Pesach Alerts by Moshe Hirsch

Quinoa is Kosher L’Pesach and is not related to the 5 types of Chometz grains, millet or rice. However, because there is a possibility that quinoa grows in proximity of Chometz grains and processed in facilities that compromise its Kosher for Passover status, quinoa should only be accepted for Pesach use with a reliable Kosher for Passover supervision.
It is also important when purchasing Kosher for Passover quinoa before Pesach to place the quinoa on a plate and check carefully through the contents of the package for any extraneous matter (i.e. barley etc.).