Ostreopsis dinos. They kill corals. I highly recommend getting a microscope and looking at the brown strings and dead coral tissue. Ostreopsis looks like lentils. Treatment is running copious carbon and a good UV for 2 weeks. I used a cheap Jebao UV ( https://www.ebay.com/itm/Jebao-55W-...841729?hash=item4601332cc1:g:w7YAAOxyeR9TGMUf ) which worked perfectly. There's a great support group on Facebook, too:
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