I think you may have more than one species given the different shapes I see. Research Ostreopsis ovata as one distinct possibility.
Many suggestions on R2R and a simple search will give you enough information/opinions for several days reading, lol. One key from my experience is to try 3-5 day blackout, add UV sterilizer and increase biodiversity with some product like Dr. Tim's, Brightwell's Microbacter 7, etc. Good idea to syphon out as much as you can and depending on size of sock you would syphon into ( if I remember correctly, I used 5 microns for catching dinos) you may want to return water to tank.
Some also recommend dosing hydrogen peroxide.
Good luck, they can be a challenge to manage.