I'm tagging along on this one. I know exactly what you're describing, I've seen it a time or two, but I'm not sure what may be causing it.
Do you know your parameters?
It's not a hard coral - in fact it's not a coral at all - it's an encrusting gorgonian, so its needs are somewhat different than corals, but typically the only thing that bothers them is low alk, and that can cause them to stay closed up, even for a days or weeks. Even at that, they will bounce right back once the issue is corrected.
If it's brittle, it's probably lacking something, but I don't quite know what.
Jenn