Whatever it is, it is a photosynthetic gorgonian. (Brown polyps as opposed to white or coloured). My semi-educated guess would be Isis huppura or maybe a Rumphella sp. gorgonian.
http://www.reeflex.net/tiere/2414_Isis_hippuris.htm
http://www.reeflex.net/tiere/2059_Ru...a_sp.%2002.htm
It may also be a Briareum sp. (corky sea finger), it's a fairly common photosynthetic gorgonian in the hobby.
http://www.reeflex.net/tiere/2060_Br...tinum%2001.htm
But the reason I think it's likely #1 or #2 is because you said the branches are thin, corky sea fingers tend to be pretty stocky (1/2" or more wide branches, at least I think so).
Not real 100% on the ID's, I just look at pictures a lot and those are the ones I think most closely resemble what you've got.