I have never seen one do this before. It looks kind of like the sweeper tentacles that some corals have, but I didn't think mushrooms have those. It doesn't appear to be dying, because it looks totally normal today.
If it was sweepers, I would think this one would be doing it, since it is being encroached upon by the elkhorn Montipora.
If it was sweepers, I would think this one would be doing it, since it is being encroached upon by the elkhorn Montipora.