I would say that there is not enough ventaliation at the crown of the plant. Also you did not say the color of your roots. If possible get a fan going so it circulates the air at the top of the buckets asap. The Hydroton is turning green cause the light is hitting nutrient rich water and causing algea to grow. Add 30ml H2O2 to kill what ever is growing on the 'ton and repeat 2 days later. That water hits the green stuff and goes into your res.