Corals can recover from just one cell. It might not look good, but if its got flesh on it, its got a chance. I have a frog spawn that had two sickly looking heads. Nedless to say, the heads died, but there are now like 20 new baby heads growin.
mabey as long as a bacterial infection does not set in and your water chemistry is stable and top notch the biggest thing is bacteria killing it off
and not all coral can recover from a single cell, although many can and do i dont beleive brains do very often
best of luck my friend