So this black algae took over my tank(rocks).
No snails would touch it, in fact they avoided it and quit even going on the rocks at all. Tangs wouldn't eat either. It was even killing my corals (acropora) from the base up. After searching every LFS for months(winter time) I found a urchin(have 2 now) that in a couple of months has eaten 90% of it except in the cracks of the rocks it can't get to .It really is black(or very very dark dark green) under white light. Since I can't take a good picture of my tanks no matter what I do, here are some pictures of other peoples tanks with said black algae. It's on the magnet in the first picture and on the right side of the tile in the second picture.