Looks like a slime mold, possibly Physarum polycephalum. Slimes start out as single-celled organisms, then get together as a multicellular plasmodium (that's the stage you are seeing) then later look fungus/mold like when the produce spores. They don't seem to do any harm in a dart frog tank. I usually get them in lab (on purpose) when I take old bark and keep it moist for an extended period of time.