Are you feeding it? It looks either nute burned or has a deficiency going on ... I am leading to deficient. Get a good quality veg fertalizer and feed at 1/4 strength. What kind of soil? If it is MG then perhaps you do have some burn issues...
I would transplant and bury that stem at least 3-4 inches if you can. Along with that, start some LST to keep the plant bushier. It is definately stretched, very streched... how is it still standing? get the lights within 3 inches of the plant, and make sure to check daily to avaoid light burn...