i doubt its mold...its probably algae. algae needs 3 things to live

1) Algae spores (good luck getting rid of those they are everywhere specially in basements)
2) Light
3) Constantly moist conditions

The most effective way to fight algae (Specially if still in seedling stage and growing in peat cubes, rockwool, or any other type of plug) is to stop light from reaching the moist plug. use some type of foam or 'light blocking' material and cut a slit in it for the stem of your plant and cover the top of the plug.