It is protein from the DOC - dissolved organic compounds.

Unfortunately, there is no test to find the same and your filters are not capable enough to remove the same. Froth/Foam is a sign of too much protein or decaying organic matter in your pond.

Overfeeding also increases the chances of foam formation.

One effective solution is to increase the frequency of water changes and good pond maintenance will help. Pond Floor shall be vacuum d at regular intervals making sure that no debris is laying on the pond bottom.

As additional measures, You can add a protein skimmer/ foam fractionator to accelerate the removal.