It is a brown jelly infection.........VERY bad. It is difficult to completely get rid of on Goniopora once it gets down into several corallites, but it is worth trying. Either purchase some Seachem "Reef Dip" or Kent "Tech-D" and treat with that (use double recommended dose) OR try to find some Lugols Iodine (at lfs or
[hr]
) which is a concentrated iodine solution.
If not taken care of super quick, the brown jelly will completely strip your coral of tissue in a couple of days (or sooner).