Bubble Popping in Marine Aquaria
The effects of this bubble popping process, if not the mechanistic details, are easily observed in an aquarium, where many things may cause a bubble popping effect. One cause that most aquarists encounter is oil from their hands. After reaching into a saltwater aquarium, skimming action often comes nearly to a halt as bubble popping dominates foam drainage and collection. The popping will proceed until the oil is somehow removed. Among other ways, oil can be removed by splattering it above the foam height in the skimmer, being foamed out bit by bit, being emulsified into the general foam as very, very tiny droplets which no longer span air bubbles, becoming attached to solid objects and removed, being consumed by tank microorganisms and by eventually dissolving into the bulk tank water. Many foods used by aquarists have a similar effect on skimmer bubbles.
As an aside, the bubble popping action of hydrophobic oils is exactly how most anti-gas medications for humans function. Simethicone is really polydimethylsiloxane, which is a hydrophobic polymer liquid. It pops bubbles in your stomach or intestine, and permits the gas to be eliminated. Antifoaming agents also are the basis for a large number of industrial products that work on the same principle. Other things also cause bubble popping. One of these is the fatty acid supplement Selcon. It causes bubble popping in the same fashion as skin oil droplets. Hydrophobic solid objects can also cause popping. Fine particles of activated carbon, sand, inorganic precipitates, or granular ferric oxide/hydroxide, once coated by organic compounds, can serve to break foams in a manner analogous to the described for liquid oils.
It would make scense b/c one would assume that the GFO is removing certain phosphates that the skimmer might otherwise skim. I would not worry about it b/c obviously the goal here is to remove by any safe means any undesirables in the system. However, what you should do is try to make sure that the water being fed to the skimmer has not been treated with any carbon or GFO and is only mechanically filtered before it is skimmed. After the water is skimmed then run it through carbon/gfo. This way you will get the most out of your skimmer before you further filter the water with carbon/gfo. This approach will also make your carbon/gfo last longer because the skimmer will first get a crack at removing stuff from the water before the skimmed was is treated with carbon/gfo.
My filtration works like this: Display to refugium, refugium through filtersocks to skimmer, skimmer to carbon, carbon to gfo, gfo to uv, then water is returned to the display.
I do realize that phosphates are not directly removed from skimming. But as you point out, GFO does bind with other organics besides pure phosphate. As such, I would think that it would be best to give the skimmer first crack at removing as much as it can to get the most out of skimming before running it through GFO b/c the GFO will bind with other organics that could otherwise be skimmed out first in addition to phosphates which may make it less effective at removing phosphates and/or make the media loose its effectiveness more quickly.