It probably will be the protien in the water from plants breaking down at this time of year, not necessrily over feeding as you are on one small feed a day. Some times a form of algae can grow in ponds that also causes foaming but i doubt its this as its getting to cold for algal blooms. May i suggest a water change? Try doing one or two a week for it and see if it starts to cear up a bit.
The foam isn't exactly that bad, it only occurs around where the stream connects with the pond. And seeing how I have a 2200 gallon pond I dont think its worth doing a water change. Thats for the input though. Apparently my brother has noticed that one of the lilly has had a final burst of life and tried to throw out annother flower but it has failed so I guessing your right about the rotting plant life