First of all, your pond is too small for all those koi. The "standard advice" is that a pond that contains koi should be at least 1000 gallons and that you should provide at least 300 gallons per koi. I know they are small now, but they become big very quickly. If you keep the same number of goldfish and cut down to two koi, it should be workable. This is not the
cause of your green water, but it certainly helps to keep it green.
Secondly, you are seriously underfiltered. A good rule of thumb is that the pump should claim to turn over at least 2 times the volume of the pond per hour. Your pump claims less than half that. You have a good pump for a 350 gallon pond. The filter claims to be suitable for "1000 gallon max pond size." What that means is that this filter is inadequate for any pond over 1000 gallons,
even if it doesn't have a fish in it. While this is not exactly false advertising, it is certainly intended to deceive, and virtually all of us fell into that trap when we started. The company's more detailed description says the filter is suitable for ponds from 250 to 1000 gallons. The 250 gallons is for ponds with lots of fish like you have. Underfiltration contributes to green water.
I agree with the recommendations others have made to aerate the pond, to plant some water lilies, and to filter the water through quilt batting.
I disagree with using anything other than covering the pond when you already have a heron visiting. If it is physically possible for that bird to get the fish, it will do so despite noise, water, or other discomforts. These all might work on a bird that has never fed from your pond. I actually find the grid interesting as is, but if you paint it black, it will be quite inconspicuous.
My additional recommendations are:
decrease the number of koi, this can be by a few each year if you can't bear a huge rehoming;
get another pump as large or larger than the one you have now,
use that pump to feed a DIY barrel filter or, better, a bog filter.
Our addy1 is the web guru of bog filters, and many have followed her example, so you can get plenty of help here. I have never heard of a pond with a properly-constructed bog filter that has green water. Plus, a bog filter grows big beautiful plants that are an asset to your landscape.
Finally, a request. Please give us a location. Climate makes a big difference in how one maintains a pond.
