If the Duncan is still salvageable, you want to get the Brown jelly off. But Do Not spray it off in the tank. It can get on other corals and cause issues. Take some water out and then take the Duncan out carefully. Once in a bowl or bucket of tank water, use a turkey baster and blow it clean. And then for a time consuming, but wort it task. Take SUper Glue and cover the infected areas. Then you should be good to put it back in.
But again, try not to get the jelly on anything else.