Originally Posted by ExoticPocket
I honestly have no clue... Has the rock been put in the tank recently? Maybe there was something on the surface and when you misted it it combined into the slime... Just a thought.
If I follow the OP's post correctly that is a piece of wood and not a rock which is why it could be some odd member of the fungal group as well as some form of bacterial or cyanobacterial growths. I don't think we're going to get a resolution even to a general identity, without a microscopic view.