That looks like a lot of material. Does your press run 24/7? It would take some time and a lot of fountain solution consumption to build up that much material if it is coming from the fountain solution, but it is not impossible. If the fountain solution contains materials that are not water soluble (or non-water soluble ingredients inefficiently coupled) material can collect in the fountain solution, but the materials you identified early in this thread would not float, but would more likely collect at the bottom of the trays and circulators. You could try titrating (adding bit by bit) fountain concentrate into water, at the same temperature you run your fountain solution, in a glass jar (beakers are good for this but not usually available in the pressroom). It may be that at certain ratios of fountain solution to water not all the ingredients will go into solution. Materials that will collect on the surface have to either be light enough to float or have a specific gravity less than that of water. I have seen instances of silicone defoamer collecting on the surface of fountain solution in circulators before, but certain non-piling additives or solvents and surfactants have poor water solubility and might contribute to this.
Daniel T Roll