When I was doing this I had similar problems, including brown algae growing inside the spraybar. The inside stuff would partially dam the flow and cause the tiny CO2 bubbles to coalesce at that point, ruining the CO2 mist effect. I solved that problem by getting rid of the spraybar. I switched to just sticking the CO2 tube into the inlet grill of my little pump, so the pump rotor would chop up the bubbles into microbubbles. That worked great, but the inlet grill would get stopped up by plant fragments, roots and snails too easily, causing a big drop in CO2.

Now, I have a bigger powerhead, a Maxijet 600, with a sponge filter on the inlet, and the CO2 tube stuck thru the sponge into the sponge support grid. For a couple of weeks now that is working great. I have it mounted upside down in the tank, with the sponge on top, so I can remove and clean the sponge more easily. This is from a suggestion on the Planted Tank Forum.