Gave up, did some other stuff. Then later in the day I logged in, not in any group, not even in the ring where the boss was. I was standing around the edge where the vendors are and saw a fight in progress. I targeted the boss and it was green. So I jumped in, smacked it around, and when it went down I got credit and loot.
So, what the... ??
I thought it was like the other world bosses, if my faction has the tag I can just wade in and hit it and it'll turn green and drop loot for me. Has not worked at all so far for the Celestials. And how I got it the one time I did is a mystery as well, boss should not have been green to me when it was.
Also apparently armory or something is all screwed up, none of my mains are showing up to select for forum posts. The Worgen that's selected is a really old alt that I haven't touched in ages, and there's one toon in there that isn't even mine at all.