If the white thread is the bobbin thread, then it looks like too much of the green top thread is being pulled to the back. Ideally you want the green thread showing on the top and only white thread showing on the back. If you can manage to get them perfectly balanced for your zig zag, you are a genius. I usually match bottom and top thread for this reason.
You can get a more balanced stitch by either tightening the thread tension on top or loosening the bobbin tension. Try setting the top tension back to the factory setting and look at your stitch. Tighten a little at a time and try samples. If that doesn't fix the problem, you may need to adjust the tiny screw on your bobbin case. Before touching the bobbin case screw, take a picture of it's position so you can return it to the original position if it doesn't help. The tiny screw on my machine goes tighty righty and lefty loosey. Make sure to adjust the screw over a bowl so you don't loose it. Good luck.