why does my plant leaves get this dirty brown looking stuff all over them? Is this algae? If so, what causes it? Am I leaving my lights on too long? right now they are on for about 9 hours. I try to rub it "off" and some comes off. I hate this; from far away you can't notice, but if you get up close and look, it looks horrible and dirty. How do I get rid of it??? :1zhelp: