Seems i have this issue also, see pic!
Its really only on two rocks, the largest of course, and two smaller tonga branches.
ive been batteling this for about 6 months now! Seems to go away alittle at a time, but still hanging around and comes back with a vengence!
You think that i can remove the red cover rocks to a QT and rid of it that way?
No light, no food, skimmer and H20 changes!
What do you think?
Algea problem
RK