You have a PO4 problem. You can manually remove it but it will grow back and spread if you do not correct it.
You need to buy or borrow a low range PO4 meter. Check the level in your tank and from your RO. Feed less and start running GFO if your not already doing so. I ran a product called PO4x4 with good results.
lanacane, I believe skene was actually trying to help.
You need to check your parameters, such as phosphates, the age of your bulbs may also be contributing to the problem.
My tangs love that stuff. Anytime I bring home a coral with some algae on it I place it on the sand bed, and the tangs take care of it for me in a few minutes.:bagfish: