I've had that in the past, urchins will eat it as a byproduct of the coralline they seem to prefer. What worked for me, ultimately, was lowering nutrients. At the time i was using the Fauna Marin Ultralith system. It disappeared in about 6 months. After quitting the system small patches have reappeared but seem to be limited to those two patches. It has been several years and a tank upgrade since I had it. Try increasing your water change frequency and feeding less (ie. lowering/exporting nutrients) and it should slowly start to recede.
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