Can't view the picture for some reason but it sounds like cyanobacteria. best way to get rid of it is just eliminating nutrients in the water, however that is vague and I've always hated that answer. In tanks i've worked on i've used chemiclean and it gets most of it on the first run if it is really bad and after a second run it is eradicated 99% of the time. It's reef safe if i remember