The infection you have, which is very common with Octocorals(soft corals) is what is known as black band disease!!!
It is basically caused by a bacteria NOT an algae! Cyanobacteria is responsible for this and is a result of too much organic food found at the base of the soft coral. This disease is especially synonymous with Sarcophyton species.
Elevated phosphate levels are more th cause of this than nitrates as high nitrates will normally effect you pH level IMO. _________ 125gal.w/Mag9.5 return(dual megaflow)>Mag7 pump Aqua Cev180skimmer.Wave2k Hamilton Reefstar(2)250watthqi(mh)pend.a Yellow, Naso Tang Red Lip Blenny Percula Clown Demoiselles Niger Trigger F. Wrasses Ceriantharia Orn.Shrimp and Stars Hermits Queen Conch asst. snails> Stars Zoos shrooms Montipora Brains Gorgonians Favia Turbinaria(large+small polyp) Acropora Xenia Tridacna (CroceaMaximaSquamosa) Leathers <35+75gal.reef tank as well> 
"IF THE PHONE DOESN'T RING...IT'S ME"  jb  |