Dont sweat it, you just have a blush stemmed plant. At one point that was how I was able to tell the two strains I was growing apart from each other. They come in a few shades, a little blushy reddish, sometimes all red or even some purple. I'm probably gonna screw the word up so maybe someone else can correct if needed... anthocyanacins. All cannabis plants have them just like all people (with the exception of albinos) have melatonin pigment in their skin that expresses in different ways. And sometimes environment can trigger it, like the end of grow season when the weather starts turning cold overnight. Since you have at least one other plant with a green stem, you know it's not your environment. You're good. You just got yourself one of them colored plants.