The wood is a black sea fan. That's what happens to it over time. Maybe since the opae are always grazing on it the black skins wears away. You would have never known that that's what it is.
Only a quart. There's probably over 50+ opae in there. Too hard to really get an estimate since they're always going in & out of the lava rock. Pics aren't too good since I was specifically trying to get the berried female and she kept moving.