Hello whitevillian and
to TR.
I think that you have a long tentacle anemone
Macrodactyla doreensis. Here is a good article
Macrodoreensis about them. They require a deep 6" fine sand bed where they will plant their foot. They also require lots of light. How long have you had it? What are your water parameters?
Moving a lot means it is not happy with something, no place to plant its foot, not enough light, poor water conditions, etc. The brown growth could be sloughing tissue maybe from being irritated from moving around or if you have a coarse substrate.
