I have had this piece of rock for 4 years, and suddenly this pops up
Halimeda (Cactus Algae) are generally hardy in a marine aquarium, and few fish will feed on it. It is not an invasive plant that will cause harm to close neighboring corals or invertebrates. In addition to light, it requires a sufficient level of calcium to grow. They are not as tolerant of high nitrate or phosphate levels compared to some other algae. They also do not tolerate extensive pruning.
it could also cause havoc with your calcium levelsit requires a sufficient level of calcium to grow
How fast does it grow?
The fact that few fishes eat it and they do not tolerate extensive pruning rings alarm bells for me.