Heliofungia "blowing up" is a normal occurance to enable them to move around the substrate (which is meant to be the best place for them, they can do well on live rock but you need to have other live rock or similar surrounding them or they will move & fall).
They are supposedly hardy...
I'm not an expert but it sounds terminal :( . I also have a heliofungia doing exactly the same thing. My water conditions are all fine & it was doing very well for about 1 week. From what I've read, once they start becoming detached from the skeletons, their demise is only a matter of time...
