Yeah I thought you might have a protien skimmer- you are positive its in tip top shape? Assuming that it is bubbling correctly and dark enough then, there has got to be something else feeding that nasty microalgae. Like the wrong light spectrum, old bulb, or phosphates in the water. What do you think?
I have a rabbit fish that loves gobbling microalgae from the liverock and powerheads (leaves it clean) but I think we should figure out what's causing it to overcome your coraline. That way you don't have to tear down your tank and start over.