Please Help Me

I was checking on my fish today and noticed that the pond water is so green that I cannot see more then a few inches into the pond. The pond water is like peas soup and there is green stringy algee all along the sides of the pond. Is there anything that I can do to fix this,especially since it is winter time. I can see two of my fish when they come up and stay next to the heater the other one I cannot find and he is usually the one that hides.

My pond Info.

Pond Size 200 Gallons

Filters: 2 bioforce filters one with a UV Clarifier

Waterfall

Fish: 1 Koi and 2 Gold Fish

1 Pond Heater

1 Pond airator

Here are my Pond Settings:

Nitrate: 20 ppm

Nitrite: 0 ppm

Total Hardness: 150 ppm

Total Alkalinity: 80 ppm

PH Level: 6.2