My baby is 5m old was mixed fed till 3.5m and now on formula his stool turned from yellow mustard to dark green do i need to worry?

No worries: It is absolutely normal for the stool change colors. The color tends to reflect diet as well as the changing bacterial environment that is taking place (very normal in babies).

No worries...: Stools come in a variety of fun colors... red (without having had a red food) or white would concern me. All the rest just add to the fun of the diaper change ;)