Sure looks like Ostreopsis heptagona to me. I’m not super knowledgeable about the difference in treatment between the different strains, but on my 40 breeder it started because my phosphate bottomed out, and I believe my nitrates were also very low or zero. I ended up having to dose phosphate for like a month just to get a reading. Once I finally got a reading, it was another couple weeks of fine tuning what and how much I fed to maintain a level between .01 and .03 phosphate. Once I got n03 and p04 where they should be, I turned the white lights off completely, and ran blue only for a much shorter light period than I was doing before. I think I went down to 5 hours a day of only blues. 2 weeks later, it is completely gone and the tank looks great again. I’ve heard of people doing complete blackout for like 5 days and that supposedly works very well. I personally didn’t want to leave all the coral with absolutely no light, so I opted for reduced lighting and it worked.
Now that my params are in check, it shouldn’t come back now that the lights are back to normal.
