I see you didn't added any fishs and that's good thing. First, your tank is not cycled, so you may have 0 nitrates, but some ammonia or nitrites.
Ammonia/nitrite is excelent nutrient source for monocellular green algae. If you're new to aquariums, then you have to know maybe you don't test correctly your water. On your nitrate test, you have 2 or 3 bottles. If you have 2 bottles, then bottle#2 must be shaked, if you have 3 bottles, then bottle #3 must be shaked.
By shaked, I mean shake it like as if your life depends of it, bang it on a table... Then you may have better chances to have a reading.
Green water is caused by excess light most of the time. Do you run a light timer? How much light/day? What's your light fixture ? Is your tank exposed to direct sunlight during any moment of the day ? Green water tend to eat nitrates very fast.
If it's you're first tank, then you're a noob to the hobby, and I will recommand you to read about the nitrogen cycle, that's one of the most important thing in the hobby. That's just general advice to new aquarist
If your tank have no fish, then just do a 100%WC... Dont rinse filter media under tap.
Note that green water is not harmfull for fishs.