Good question djmuzzi, wouldnt be so quick to dismiss it yet waterfaller1. What waterfaller1 is assuming is its expelling waste through it's mouth, hence djmuzzi's question for more specifics. If the mucous is coming from the tissue or is on the tissue then theres a serious problem. If indeed it's coming from the mouth this is a natural occurance that shouldnt cause alarm UNLESS it's excessive and occurs frequently throughout a week or more.
The brown slime that covers the tissue may be an infection. It is referred to commonly as a "brown jelly, or brown jelly disease,infection, etc.". Many variables can cause this. To happen overnight makes me think of another coral may have come into contact and zapped the hell out of it in the process(most common), or some predator or excessive nipping?
If indeed an infection, act fast. Try dipping the coral in an iodine sollution, to cure the infection, then place in a low flow and lit part of the tank for recoup. If its declining by the hour, it may be to late but you can attempt to save any tissue and skeleton by fragging it, if you do, only save the healthiest portion and clear off as much dying tissue as possible.
Ive had no success when it gets to this point but some friends have had better results when cutting at least an inch away from the dead tissue.
Good idea to seperate it as far as you can from any other corals as not to get them infected as well if indeed it's an infection.
-Justin