Threadfins have a pretty medium reef-safe rating as well and ironically I had heard they were hardy. The problem is all butterflies are pretty delicate compared to other fish though, they seem to be among the first to kick off at the first sign of trouble.
Sorry about your loss there Mike.
FWIW, I had a raccoon butterfly over the summer months until that weird clam spawn incident (?? I'll never really know for sure) in September in which I lost him, my flame hawk, my rabbitfish and my sixline in the space of a few hours. Anyhow, he never once bothered the clams. Having said that though, he did do a fair bit of damage to the LPS corals and didn't judiciously eradicate the majano anemomes to the same extent that the ulietensis have. He must have been picking at them though as their numbers quickly jumped after he died.
But comparatively, the pair of ulietensis I have now pretty much removed several dozen stands of majano within a little more than a week. Within a couple days, I was sure there were fewer, and within a few days more I actually saw them picking at them .. and within a month they were ALL gone, even the ones that were hard to get at because they were in crevasses or under clams.
What you have pictured there is the falcula - notice the tightly defined triangles for the black spots. Whereas ulietensis the black is more gradual and more like stripes. This is ulientensis:
Here are mine:
I am going to have to take them out at some point, it breaks my heart to do it because I love them to pieces, but they are picking at my gorgonians. Gorgonians are about my last stand with corals these days and to start losing them now I might as well switch to FOWLR and save on electricity bills for the lighting.
Ultimately I might setup a FOWLR just for butterflies though. After seeing some incredible semilarvatus recently I'd love to have a FOWLR with some butterflies. Then I could just plop some aiptasia or majano rock in there for a cleaning and poof done problem solved..