Why is my rockwool green on top?

NO man it doenst have to be that exact. I use a spray bottle, fill it 3/4 of the way with water and fill it up the rest of the way with peroxicde.

ok cool, thanks for your help!

Glad i could help but you should do some reserch on white mold or powdery mildew.
Use the search on here.

Are you using green top brand? I heard thats some quality stuff man keep up the good work.

Yep, algae. It's caused by oxygen and light exposure to stagnant water. Blocks access to air for the roots; causing suffocation. Can be prevented by not exposing the possible place for algae to grow and can be eliminated easily by spraying it with hydrogen peroxide.

The Green algae will not cause a problem by itself, but what happens is mold start to feed of the Algae and then you have mold spores spreading around the room by your fans.