How to prevent green algae from forming on the surface of coco?

"If you see green stuff growing on the top of your coco (algae) it’s often a sign that you’re watering too often. Although algae won’t hurt your plants directly, it only appears when the top of the coco has been constantly wet for a long time." https://www.growweedeasy.com/how-often-to-water-coco-coir

Block the light any way you can. The green algae is pretty normal and not really harmful, but you're right, it is annoying and it does use up some oxygen.

You just need to block the light to the medium. You can use something called Panda film for a commercial product, or something as simple as a piece of tin foil.