Machlokes Rav Belsky and Rav Heineman l’havdil. I do believe that one of Rav Belsky’s main concerns was that it’s grown near grain. The Star-K puts a hechsher on some quinoa and I think the majority now paskens like Rav Heineman.
Corn is different because we have a near unanimous consensus by early acharonim that it should be considered kitniyos. So we (Ashkenazim) don’t eat it because of mesorah.