I have two of them, and they get varying degrees of algae on them over time, but not to the point of disrupting the vortex flow. I'm guessing the CO2 reactor probably isn't the only thing periodically becoming dirty in your tank.
That said, I did have one of the pumps just die on me recently. I was having trouble with my CO2 needle valve, and I assume the pump filled up with CO2 and ran dry for too long, and it just burned out. They're great little reactors, though. I often recommend them as a compact CO2 diffusion solution.