My experience is that euphillia are super sensitive to elevated phosphates, and start shrinking/retracting their tentacles in water that has too much phosphate for them.... then they just start dying.
Usually when you lower phosphate, they bounce back pretty quickly. You said you're running phosguard. Have you changed it recently? With high phosphate, it can deplete your media pretty quickly sometimes. And for whatever reason, phosphate tends to persist through standard water changes.
So... my suggestion is one of two things: replenish your phosphate media. Get a phosphate test kit and watch the numbers.... you want it down to .05-.10. In my 90g I had to use a bunch of gfo (almost twice the recommend) and needed to change it out almost a week after I had put it in. note that this did kill my chaeto, and that prolonged the phosphate spike.... so if you have microalgae, keep an eye on it and be prepared to remove it.
2. A series of large and frequent water changes. Aim for 50%, every two days for a week.
Once you have it under control, you might need to add a small amount of phosphate media, to help keep phosphate down, as generally, many ways of exporting phosphate like macroalgae, tend to remove more nitrate than phosphate comparatively. So over a long period of time, phosphates will creep up, and your euphillia will start looking bad again. In fact I keep euphyllia precisely for this reason, as it tells me when to change my phosphate media.