Your pond is starting up its ecosystem again. Almost everything goes dormant during the cold months. As the temperatures begin to rise, algae may form and cause green water. Because the plants have not grown back or new ones have not been added yet, the algae has no competition for food sources and thrives. It will balance out naturally. You can do a partial water change, add beneficial bacteria and use algaefix if you want the green water to disappear quicker.
