Although some will argue that green water is healthy for goldfish, and this is true to an extent; it actually depletes oxygen levels by taking up space. You'll find green water in ponds and tanks with very little surface action. Increasing surface increases oxygen levels which eliminates green water.
Add a pond pump to your set up; they're affordable and increase surface action without creating a current in the body of water. I'll bet you've got a top loading filter in your tank. These types of filtering systems are actually designed for tropical fish, not goldfish. They produce little surface action without creating a strong current in the body of water.
Pond pumps sit on the bottom of the tank where the goldfish waste is; offering a richer food source, which increases your friendly bug colony. Increased surface action reduces harmful bacteria and eliminates supersaturated gases found in tap water.
You can find water treatments that eliminate the green water temporarily, and you can even block out the light which works temporarily, or you can use the UV sterilizer which only works temporarily, but if you increase surface action by adding a pond pump to your tank, you'll never have to worry about spikes to your nitrogen cycle, depleted oxygen, low pH, harmful bacteria or green water ever again.
It's also very important to flip your lid on your aquarium exposing the surface to fresh air.
If we're going to keep these pond dwellers in aquariums, we've got to make some compromises.
Check out my article on 'Algae and Green water' GoldFish Emergency 911
Best of luck and may your tank water never turn green, Venus