Foam in the filter or in the pond is usually caused by pollution from high Protein levels in the water by the use of high protein diets or dissolved organic compounds building up through the metabolism of koi, or the turning of waste products into solution. These organic compounds are not broken down by the bacteria that removes the ammonia, nitrites and nitrates, but by oxygen consuming heterotrophic bacteria These arnt able to completely breakdown the organic compounds therefore pollution levels in the pond will rise and poison the fish.
Test the water perimeters as you may find they are at high levels especially this time of year where your filter is not functioning due to the die back of the bacteria, stop feeding immediately if you are still feeding, and carry out water changes to dissolve any poisons, if your pond isn't heated then you wont get the bacteria back until the water warms up in the spring. Foam can occur with new pond syndrome too due to lack of bacteria in the new filter. You could also fit a protein skimmer to remove the organic compounds from the water before they break down into nitrogenous waste. As a protein skimmer will physically remove organic compounds before they begin to decompose, so lightening the load on the biological filter. | 
