Well...i would not throw away the plate...actually i cant really tell if that is the brown jelly.
here is a bad case on my colt...i had to amputate all of that.
Maybe try another iodine dip on the plate...but for soft coral just siphon it off then frag the infected part...colt and xenia are very good at coming back...my colt is already growing polyps at the cut site.