It's a very finicky coral, but very beautiful. I put a frag of a purple one in my tank this past February. It had three polyps when I added it, and it looked healthy for about a month. Suddenly it stopped opening up, but didn't look dead. I finally saw it opened up again over the summer, and now I find at least one or two new polyps growing on it every week and it's now about 2-3" across.
Nothing changed in my feeding and maintenance routine or in the water parameters that would indicate why this coral suddenly wouldn't open and then suddenly stayed open.