My guess is most likely bacteria infection, which is more than common for euphyllia. This one might be beyond saving imo. Hopefully the diseas hasnt spread to others. I would recommend removing it from this system if you can to prevent further contamination.
My torch had half it's polyps bail over a few days because of a change in flow I believe. My return nozzle usually rotates in a circle but got stuck and was sending a stream of water right at it it was a day or two before I noticed. Now he's doing better. I didn't move it or dip it and it's going great again now for it to grow over the dead polyps.