It could also be from your dechlorinator - often the ones that contain aloe or other plant products for slime coat will do this.
Inlet and outlet hoses will get gunked up, usually with diatoms but sometimes with light-colored bits too, so it is a good idea to use a hose brush from time to time to maintain good flow. Nothing to worry about.
That's a pretty cool surface skimmer, Vega, and would do the trick in this case since it does not disturb the surface. Normally I recommend aiming the spray bar across the surface to disrupt the film but when running CO2
it's not the best thing, and this solves that problem.