I have a question about the cause of a layer of green on top of the soil of the plants I've been taking care of. I'm working in a greenhouse and was in production most of the year and a few of our watering staff was laid off and now I'm watering 8 hoops that have plants in all stages of their growth. They all have a layer of green on top of the soil making it difficult to water and tell if the soil is dry or wet. None of the other hoops at the greenhouse have this problem, even though it's all the same soil. What is the cause of this green layer and what should I do to correct the problem?