i recommend taking the rock out, and "superglue-gelling" the euphyllia to the rock OUTSIDE of the tank. once the glue cures, if you did it right, should never fall out.
i find over time epoxy gets weak, starts to crack, and then bam its broken. also gobs of epoxy in my opinion is tacky looking and a plain eyesore, even after it becomes encrusted with algae. if you decide to use epoxy, get a piece of rubble, and make impressions in the epoxy with it (like a stamp) to make the shape of the epoxy more natural.
the only downside to using superglue gel is , you need a LOT of it for larger corals and i mean a LOT. could get kinda costly