duncan coral slime

I have two heads of the duncan coral. Last night I looked in the aquarium and found that one head had brownish/white slime on it. What could this be?

 

slime vanished.....do these corals tend to shed?

 

All corals slime to clean themselves off. Increasing the flow in the tank will help prevent this from happening as often. LPS corals like duncans need no more than moderate flow going past them otherwise the flesh could be torn from the skeleton.