I just went through a bout with this white fuzz on my zoas in Frag tank . I tried every dip imaginable including peroxide looking for something that could effectively kill it. I never found any dip that kills it , but they would help temporarily and it would go back to looking bad in short time . Brushing them seems to help a bit . It seems to have just started to disappear on its own after a couple months since I first spotted it . Honestly have to clue why it’s getting better now but thought I’d share