Given the amount of buildup where the tube connects to the PCV from the filter assembly, I imagine there is much more at the lower end of the tube, and inside the primary separator. You can clean the tube when removed, but the separator would need to be removed. And still, after soaking it, you wouldn't know how clean it is inside for it being a sealed unit. I would just replace it. For the need of new seals on the filter assembly and cooler, I replaced my filter assembly (seals were included). But each to their own.
As said, when the crankcase venting becomes clogged, it can develop air leaks from the pressure forcing its way out of the crankcase.
"What to do next"...in my opinion is (some of this is in assuming it is all foreign to you): some cleaning of the crankcase venting bits as discussed, drop the oil pan/sump and clean the oil pickup screen and clean the pan, replace all leaking seals, look into self-checking the cam follower, checking the timing chain and tensioner health, and studying some general TLC practices of others. Whenever you have something of concern or if you are simply interested in something...search for "MK5 GTI ______" ←*fill in the blank* and read threads/topics. Lately, it seems car manufacturers build cars for the consumer mindset that they are disposable. And only intend for the car to last beyond their warranty periods or to last the period of time people own a car before replacing it. Ours didn't even include the cam follower in any of the service intervals as an inspection in the owner's manual - service interval booklet.
