Its coco & just dabbing a little of mixed h2o2 on top after scraping off the Green algae would fix this issue, Just a little dab over where youve scraped the G'A off is fine & wont do too much damage!
you could also use physan 20, but like the h2o2 it will affect any other biologicals in your coco. if you're just running straight chem feed, though, it's no worries!
Your soil is alive and supporting algae and microbes. That is what you want. If you are using natural nutrients then the algae and microbes break down the nutrients and turn them into sugars and good stuff that your plants love. H2O2 will kill it but why would you want to do that?
If you have damp soil you will get gnats with the alga or not. I sprinkle tobacco on top of my pots at first sign of gnats. Gnats gone and my soils biology kicking ass.