I got a quick response from Ron Shimek...
Hi,
You asked, "Could this be Actinodendron or Phymanthus or something else?"
I think the best answer is, "Yes."
Definitely, it is a branched anemone. It could be a juvenile of a species in either of the genera you have mentioned, or in another related genus. There are probably around 30 potential species that it might be. Frankly, distinguishing these species is difficult and needs animals that are really somewhat larger. Some of the branching characteristics will not show up until the animal is bigger.
The common name for any number of these animals is "Hell's Fire" anemone, referring to the delightful sensation one get when stung. These animals are dangerous, and it is not related to size. Not only is the venom of the sting potent, but one can be allergic to them, so... plan ahead, and don't get stung. If you do get stung once, don't do a second time (one can become sensitized to the venom).
Sorry not to be of more help. It is a neat animal, and if I had it in my tank, I would do everything I could to keep it happy.
But, I like odd anemones.
Cheers, Ron