I took this picture after the ice just disappeared on this pond. Why is there so much live green algae?

The most likely cause is the release of nutrients from the organic sediments on the bottom of this pond, no surface waters run into it. Along the edge there are several large willow trees and dense growths of expired cattails.  The trees have deposited leaves into the pond for many years and the cattails have collapsed into the water. As these leaves and cattails slowly decompose they release nutrients back into the water for the growth of algae.  The best pond maintenance approach is to reduce the organic sediments and harvest the cattails during the winter.