That depends on a few things. How bad is the green, how slow is the dwell time, are there other plants, is there shade, how old is the bulb? Truthfully I am not that quick to change liters to gallons, I am pretty old and not in the sciences but I got 1981.3 gallons. I have used an 11W UV with a 1000gallon pond and with a slow dwell rate had great results. I would suggest you experiment by slowing down the flow rate. I would add a pint of Hydrogen Peroxide to see the results. It is possible your bio filtration has been overwhelmed by algae and needs a starter with water from an established pond that has no algae problem.
I suggest you check out Norm Meck's article for green water. In the last four pages it discusses the possibility of the lack of a particular antibiotic in the water which could be replaced by the water from the clear pond. It is possible that a commercial product could contain that antibiotic but as Meck says, it isn't known for sure what that particular antibiotic might be.

Here is a link that might be useful: Norm Meck green water