My Pond water is incredibly green...WHY? 10 points to best answer?

OK so i have a 90 gallon pond. With a filter that is on during the day and off during the night. It has ONE goldfish in it. At the begining of summer after the pond thawed out, the water was clear and the goldfish alive and happy(I had put him in aduring the summer and he survived the cold cold winter). Then as the spring has progressed, the water has gotten gradually green. Why? is this bad? and if so how do i reverse it?