I have a 10 page thread on my former bryopsis problem floating around here somewhere. Just do a search man. There's tons of them.
I cooked my rock in a dark tub for a couple months. After a month in pure darkness, some of the bryopsis was still surviving on the rock. My rock was saturated with nitrate and phosphate from years of using tapwater in my formerly FOWLR tank, that wasn't maintained the best. I purchased new rocks here and there and added to the pile but all the original stuff was still mixed in.
The bryopsis was a nightmare. Nothing would eat it. The nudis I got to take care of it were eaten immediately by my peppermint shrimp. Urchin wouldn't touch it. Paramaters were fine. All the water changes in the world won't touch the stuff.
After cooking the rock, I tore the tank down and completely cleaned it out. Dumped all old substrate and seeded new with a cup of the old. Got a phosban reactor going with carbon and phosban or rowaphos. Tried my best to cut back on feeding and raised the Mg levels a bit. It eventually let up for me.
Now I have some other type of strange, wiry algae I contend with, but at least I can pull this stuff off of rocks and equipment.