This can happen from excessive fobing (foam on beer) during your process. The usual culprits are when you oxygen before pitching, during fermentation, during carbonating for kegging, or during packaging. Any time you shake a fermenter or keg to dissolve a gas (be it co2 or o2 depending on where you are in the process) you create fobing. Anytime you have a vigorous fermentation or transfer beer from a keg to another container (serving glass, growler, bottle) with too much pressure, you get fob. The problem with fob is that a beer's foam potential is limited. So the more times you create fob before you actually want it (usually in the serving glass), the less foam you generate later on. When a beer is at the end of it's foaming potential, it's pretty common to get that foam appearance you're describing, almost like a soap scum quality on the surface. The solution is to limit fobing during your process as much as possible.
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