What is this and why is it starting to die?

[laura9797]

Thanks so much for this awesome thread! Is this coral Galaxea? Why does it appear to be dying off? Is started to spread and then I had to deal with red slime. I have been treating the slime with a new product called NO3O4-X a biological nitrate and phosphate reducer.
Any help will be much appreciated!

 

[ccCapt]

That's Briareum, a type of star polyps.