Found a few things on line that suggest this is not too uncommon in planted tanks. The suggestion is that it is debris from food and decaying plants and that it can be hard to get rid of. A few people suggested more floss in the filter to try to catch it and otherwise, keep doing what we are doing (pwc and careful siphoning). I did see one suggestion to try to blast it off the plants with a turkey baster or something so as to let the leaves continue to get light.