What is causing a white foam on top of the water?

It most likely is DOC, or dissolved organic carbons. The only way to get rid of it is to make water changes (i.e., 5% per day until it's gone) or install a foam fractionator, available from Whitewater Filters or other pond supply sources. (Answer courtesy of Bryan Bateman)

Since the foam you describe does not have a rainbow effect to it, most likely it is dissolved organics that are in the water. These organics form air bubbles that don't break easily. This is the same foam you will see on oceans and lakes. If you do regular water changes of about 10% every week, you will remove enough of the organics that it should go away after a month. Or you can install a foam fractionator on your pond to remove the organic waste.

(Answer courtesy of Mike White)