all u guys have to do is cover up the rockwool with some paper or cardboard. if thier's no light than thier's no algae. i cut me some cardboard into little circles to cover the bottom of my plants cuz i was having the same problem except for the mold problems. but that should go away too. just stir some of the rockwool around.
algae lead to root rot if u allow it to get bad enough. but then again who would let it get that bad?I would, but since it doesn't hurt the plant any, I'm not worried about it, it just looks nasty.