The green stuff is probably algae. Algae shouldn't hurt your plants, but it may indicate you're keeping the grodan too moist. Not sure about your system but cut back on watering (if possible) and cover the top of the pot with a bit of black plastic. without light, the algae will not grow.

I agree the algae is not a huge deal. They sell snap-on net pot covers if you don't want to make your own.