Yea, a surface skimmer would definitely degass a lot of CO2. So will surface movement, but not nearly as much. Like Widefx says, you may need to up the CO2 to keep the same concentration.
One thing.....my surface scum is not really green. More like an oily film. Could you go into further detail about what ti looks like, it might be something different.
I would guess that much of the surface skum produced is due to heavy bioloads and lots of fish foods. Maybe lower the population in teh tank, or just the feeding schudule.