Mine got buried in the sand after the tank move. I have a few bare branches and decided not to do anything about it. Watch it closely and if the recession continues frag it above the dead part and seal it good. Hydnophora are pretty tolerant of most things, but I have never fragged one.
Ditto on Scott's thought. They are one tough SOB! If you do frag it and then seal it, make sure the piece does not wiggle or anything like that. Make sure it is a very snug and tight fit!
I would...at this time, I do not see a reason for waiting since it is receding from the bottom up. Just snap it off above the living tissue, and seal it with super glue and the glue to a rock. Good luck and keep us updated with your beautiful pics.
"When they was no meat we ate fowl, when there was no fowl we ate crawdad. And when there was no crawdad to be found, we ate sand."--Cellmate
"You ate what?"--H.I.
"We ate sand."--Cellmate
"You ate sand?"--H.I.
"That's right."--Cellmate