Bryopsis. Lettuce nudibranchs will eat it, but then starve to death when it's gone. Once the nudibrachs are dead the bryposis will return. Evil stuff. Best way to deal with it is manual removal and nutrient control.
Check your paramters. I had that a while ago and my parameters were not in check. It will go away by itself once you do that. If you want some fish that eats it my hippo tang loved it as well as a yellow tang you can go with.
Bryopsis. Lettuce nudibranchs will eat it, but then starve to death when it's gone. Once the nudibrachs are dead the bryposis will return. Evil stuff. Best way to deal with it is manual removal and nutrient control.
It depends where its located if its located on a glass or a piece of a rock that you can take out then you can scrap it off with a brush but if its located inbetween your zoas than thats a different story and its just comes back even it you use a plucker to pluck it out. Good luck.
Unfortunately it's a tough battle sometimes. Sea hares are very good. You might want to turn your lights off for three straight days. This seems to weaken it. When the forth day comes and you turn your lights back on, it seems much easier to pull clumps of it off.
Phosban, water changes and a good cleanup crew. With all these things you might be able to beat it.