I asked the same question a couple weeks ago and I think steve-s told me that if the polyps are white they are primarily filter feeding, and if the polyps are of a different color they are lighting dependent.
I was givin a Purple Sea Fan, and it is doing awesome in my 29gal under NO
lighting. The polyps extend every night and I can see some growth already! I have it placed way up high and almost directly in the current from my PH
's! It has already shed a couple times and each time it does it seems to get bigger!