The green slime you have on your rockwool is called Algae.

Algae needs light and nutrients to live, it will spread to your reservoir and will block feed lines. Because of rockwools unique vertical 'tube-like' structure, which takes up nutes via capillary action, and pulls air into the medium as the nutes drain outward, a heavy buildup of algae can effectively block this breathing action. To prevent this from happening, use light-proof and water-proof material to cover your rockwool, for example black/white panda film or thick black bin liners.