Same stuff I have. My tank has absolutely no algae, just this stuff. I thought that is was my ro/di, bought new with tds and no change. I don't have allot, but it's there and not easy to get out. I also have gfo and carbon reactor, 100% no Po4No3, according to test kit. (Hanna and Salifert test kit)
I did a complete black out of my DT and Fug for 3 days and it was gone (did it twice). After a few days it started to come back. I have a feeling it is a type of Dino. Water change does nothing.
Here’s a pic. Mind you when the tree coral in full open the stuff is very thin strands.