You're fine, those are just pre-flowers, it's not a sign of autoflowering, it's not a problem with your light cycle (Plants pay more attention to the length of the dark period[and if it's getting longer or shorter] than the length of the light period, and 16/8 would be fine even if it was the cycle you were running, if plant's couldn't veg under less than 18 hours of light you couldn't grow weed outdoors in cali, I have plants vegging outdoors right now in ~14 hours of light), it's just something they do when they get old enough to be flipped to flower, which you can do whenever, but they will continue to veg as long as you want them to. All it means is that you know for sure that it's a girl, so congrats. The stem color could be from deficiency or temperature, but it can also just be genetics, so I would look for other signs of deficiency before I try to treat the stems, if everything else is growing great without problems it's probably cold or genetics, neither of which are a real issue.