What do you mean by "a slow process like other anemone"? Anemones can live for decades if not longer, a sudden death would indicate disease, predation or environmental factors.
I've got 2 Rainbow anemones that's been in my system for at least 3 years ... traded a baby rainbow nem and got 4 Rock anemones in return. A week later none of the rock anemones survived ... go figure
I like them better than BTA, and I’m raising 8 babies, 4 orange, 4 green. They will move, but are great additions. My BTA split, and then my clown ate both. The clown doesn’t bother the rock nems