Generally water flowing through keeps then clear.
Things don't generally grow in the dark either. You sometimes get tiny sponges and spirorbid worms that filter feed in the fast, dark flow, but generally the flow is too much for them. Any "build up" would just be ripped off in the flow.
Pipes will feel slimy inside overtime, but that's bacterial mulm build up, same thing that needs to build for a skimmer to "bed in".
