Was your pond and filters running all winter? Some shut down and as a result have to go through the cycle each year on startup, and green water is prevalent during the cycle. Green water algae thrives on ammonia, and because it consumes so much ammonia, the test kits will rarely find ammonia present, which is good for the fish, not good for your viewing pleasure. A cover to keep the pond from freezing and running the filters year round will keep the bio bacteria populations much closer to the needed quantities so they make minor adjustments with changes in feeding and fish activity levels.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard