I think it's a pretty major contributor. I'd find some way to address it - esp if pine needles, dog hair etc build up in there. Another option might be to follow your p/w with Envirocare Moldex, or the recommendations at cleaningcompositedecking.com This would keep it cleaner between cleanings.

I have had my trex about 7-10 years I am guessing. I was so excited, maint. free and all. Boy was I wrong. Still looks nicer than any wood deck but need to clean mold/mildew twice yearly is a pain and I have a very large deck. Today for the first time I used a power washer on low pressure with the cleaner that came with it "G Clean", didn't do much as far as I'm concerned, for mold mildew etc, NOT. Its cleaner but all mildew/mold spores are still there so i thought I am going to try a test area with vinegar/baking soda and see what that does, pretty much nothing again. I can deal with the way it looks as the spotting doesn't show when dry so thats good but I am looking into prevention more than anything now. Any luck out there? I want to stay as organic as possible and may just do vinegar and baking soda and see how it goes. Not feeding the mold/mildew is probably better than anything. I live in MN which can be very wet at times and my deck is East facing primarily with some South sun as it passes on by to set in the West. I love everything about my trex except the mold issues http://www.maids.com/blog/3-non-toxic-ways-to-clean-mold/

Take a sample of the decking to any paint store and they will find you the closest match. Get a few samples and apply on the scraps of wood and see what will be the closest match. That being said, redwood is naturally come with darker shade characteristics so you can make a very close match.

Based on where you are, I'd guess you'd only have to oil the ipe once a year to keep the color. Honestly, the bigger maintenance headache by far will be cleaning the sap, pine needles, etc., which you'll have to do no matter what material you use. Composite decking is certainly not maintenance-free: https://thecraftsmanblog.com/the-problems-with-composite-decking/ You do have to be careful choosing a good ipe supplier with reputable sources. But as long as you have that, real wood decking is actually better for the environment: https://blog.advantagelumber.com/2014/01/02/the-environment-and-your-decking/ But composite decking may not be as eco-friendly as you think: http://www.compositedeckingreviews.org/blog/2014/01/composite-decking-is-not-eco-friendly/ You definitely need to watch out when it comes to composite decking. Some projects turn out fine. Others...not so much. Composite manufacturers have a history of making bold claims that don't hold up in the real world, then finding creative ways to dodge their warranties.