Formula fed baby's poop

What does green poop mean for a formula fed baby? My 5 week old son only poops once s day now. He skipped a day yesterday, but for the past week or so his poop has been dark green and more solid than usual. His poop normally is a mustard color with seed like things in it. He's only formula fed. Is this something I should be concerned about? What could I do to bring it back to normal? He was down to pooping once a day too when it was normal.