You can get green water without measurable levels of ammonia that would danger the fish. More of a concern may be the oxygen depletion in the water. Steps to take as suggested, are to limit light on the tank (especially direct sunlight) and make sure you are staying up on your gravel vacuuming and water changes and not overfeeding. If it is really bad and you don't have luck with these steps, it's probably time to give it a one time treatments with an algae killer, making sure you do not overdose. That should clear the tank up, then give it a really good cleaning and water change and keep up with the low light and cleaning to make sure it doesn't come back.