If after all your searches you find nothing then we can kill it with peroxide like we do in so many other tanks. If it is algae, which I can't verify, then it can also be attacked from a nutrient standpoint, sometimes. If that worked everytime, nobody would be using peroxide and the thread would have only one salesman in it lol.
But a two pronged approach makes id nearly unnecessary
use gfo and starve it and/or simply target kill it in or out of the water like we have done with a thousand tanks so far. the thread is pico reef pest algae problem challenge thread in the nanos forum page 1 or 2
one last question, have you ever manually removed it all by hand where there is no visible growth or was it just left unchecked since you first noticed it? the speed of regrowth is a helpful gauge in algae wars