I just built my first Terrarium/Vivarium. It is a 40 breeder- pond in the front corner w/ pump.... pump curculates to piece of cured wood that flows (more like dribbles, not a great pump

) onto another peice of wood that is the stream, back into the pond. The problem is that on the "waterfall" their is a slime coating.... not on the whole piece of wood just in kinda a rounded spot. the water doesnt collect their but it does flow over it. its not green so I have ruled out the fact that it is algea. it is clear (brownish red on paper towel) my questions are
What is this slime?
should I and if so how do I get rid of it?
Is it harmful to Betta, Crested gecko, or poison darts????
I have already battled the mold problem I had by adding more ventalation to tank.... before I add anymore plants I just wanted to make sure slime want harmful... or if it the natural occurence like the mold....
thank you for reading.... any helpful hints about slime or anything would be greatly appriciated