Green water is usually from too many nutrients in the water combined with your lighting.
First, is there plants and do you add any liquid fertilizer to the tank.
Second make sure your nitrates are not high.
How long are your lights on?
Free floating algae is blooming in the tank right now.
You may have to eliminate the source of Nutrients and may have to leave the light off for a day to a few days.
If it is a planted tank, leave the light off a day, and stop adding fertilizer.
You should do a 25%water change every 2 to 3 days for a week.
Make sure you are not having sunlight hit the tank.
When this clears don't leave your lights on for more than 12 hours. After you turn your lights on in a couple days ,at first , I wouldn't leave them on for more than 8. Eventually moving up to a longer time.
It isn't a bad thing if your Nitrates are not high.