I see where you have 5 LEDs but what are the wattages and what color are they. To my understanding black/dark from is from high nutrient water, you could try to lower nitrates other ways to help out the scrubber then let it take over. Also I don't see where you mention screen size but if your feeding 1 cube/day then the one sided screen should be 24 sguare inches lit by 12 watts of led which is why I ask what wattage the leds are and if the color spectrum is wrong then it will be harder to get the algae you are looking for if at all.
HTH