Stop what your doing now. It's too young in the plants life for mg deficiency & I wouldn't give any nutes yet, there are plenty in the soil for a plant so young! I dont know your soil & if its ok to use with such a young plant but To me it looks like a
strain characteristic. It looks healthy to me if that lighter green is just the picture.If anything the pot is a little too big, for a little plant like that, you can develop drainage problems later on if your root mass is loose, i pot up to bigger pots, this helps the roots to fully develop, which equals better yields. Also this way you dont have to feed nutrients untill you are in flower , as the plant is re fed at each pot up.
