How to avoid 'green slime'

Another word for 'green slime' is algae. When you have a light colored surface moist with nutrients and water, you will get algae growth. Algae are not harmful to the plant, but excess amounts of Algae and especially dead (black) algae are likely to attract fungus gnats (flies). It is best to keep algae down to a minimum.

In the stores you can buy plastic or coco block covers. Both work very well to minimize the algae growth.
If you have a bag of expanded clay pellets (Hydroton/geolite etc) you can take the dust from the bottom of the bag and sprinkle it on the GRODAN blocks to minimize algae growth.