Why shouldn't they? The Greens ~are~ another party so they run against the other parties, Republicans, Democrats, Libertarians, whoever. Rashida Tlaib is a Democrat.

Most Greens I know have strong opposition toward both major parties telling you that they are both pretty much the same, owned by Wall Street, blindly support the military, and neglectful of the environment. [Truth hurts sometimes. eh?]

They are not concerned about how they may effect the other parties. They are out to build a party, get votes and win. Just ask Al Gore. Greens decided that election.

That said I seriously doubt if they will have any effect in this race.