Sometimes one plant will just be sickly , who knows why ? Like PEPSIKOLA said , it is either overwatered or it is underwatered, or the light is too close , or it is too far away......WTF ? WHO KNOWS ?
I would concentrate on the crop as a whole , if the rest of the crop looks OK then don't worry about the one , unless it gets a fungus or infestation that could spread to the others, then kill it. I think most of us early growers tend to look at the ONE plant that looks a little strange to us and we freak out, natural reaction. The crop on the whole looks good to me.