You have an algial bloom. Blackout the tank by covering it with a tarp or other such material. Zero light for 1 week will fix it.
Follow with a 50% water change and ensure the water is aerated as the dying algae will release Nitric Oxide and CO2.
Follow with a 50% water change and ensure the water is aerated as the dying algae will release Nitric Oxide and CO2.