this algae is the devil (red turf). you can try a number of things.
a) ULN (ultra low nutrient) along with HEAVY GFO (ferric oxide) in a reactor and expect to wait a LONG LONG time to make a dent in it. this stuff survives and thrives where not even bryopsis (another dreaded algae able to survive minimally low nutrient conditions) stands a chance. NO other form of algae survives my tank. I have a low bioload, tank established over a year, HC GFO in media reactor, carbon and bacteria dosing. this is the ONLY problem and I've had much worse water quality in all my past aquariums (15yrs) and never had the pleasure of this algae being introduced before. all started from one rock with a small insignificant looking patch. too bad I didn't know what I know now.
this is it what I called semi managed from several months back and I noticed some has since slowly turned white and having a hard time growing quickly (under normal conditions it will grow and take over in no time) with time and improved water quality. its not able to pack on as heavy and thick as it is in your tank but at various points I've had a few thick areas like yours. I'm also using strictly distilled water while I'm battling it (cant risk RO membranes and DI filters declining though I have unit). next up will be a UV to help eliminate algae spurs in the water column.
b) pull each rock and manually remove it clear off the rock completely (I'd suggest a dremel with appropriate attachment so you can file down to the white meat of the live rock) THEN maintain ideal conditions so it will have trouble coming back.
c) urchin. some sea urchins will devower it but they have a habit of knocking over rocks and coral if they arent secure. some species can also get really big (8-12") so you may want to swap them out before they get jumbo and nobody wants it. if its not eating it try another species or urchin. I've heard mexican turbos will eat it too but I havent seen actual effective results (mexican turbo shells are completely different then so called "turbo" smails which are astrea snails with cone shaped shells. mexican turbos dont have cone shells and get relavitely huge). I've seen tanks that urchins completely wiped it out clean in no time.
astrea "turbo" snail (do nothing with red turf)
mexican turbo snail (supposedly will eat it)
d) throw everything out and start over. I seriously thought about taking all my corals and cutting them at the base and basically starting over.
