Hey Scott,
If you can easily see it's mouth (cant tell in the picture) it's most likely a Macrodactyla doreensis, One of the more common anemones being sold as a long tentacle. It might also be a Heteractis crispa, also regularly sold as a long tentacle but they normally have a much more dense tentacle count. Either way, both are pretty cool.
Of course I could be way off on both guesses.....