If it smells awful, I would go ahead and say it could be caused or related to decaying food matter that floats down and settles on the log. Any thing that smells awful should be cleaned because it is most likely rotting. Does it just look like slime or does it have a fuzzy appearance? I would also move the log to an area of the tank where there is more current, like right under the filter, just to be safe and make sure stuff won't settle on it. If it keeps coming back, it could be the wood rotting itself, and I would say you should remove it for good at that point. Otherwise, it just sounds like maybe some food or other organic material has built up on it and begun to rot, so I would clean it off immediately, just to be safe, because if it IS rotting food, that can mess up your nitrates.