Could be Downy Mildew. Let the medium dry out and keep it elevated. You don't want any type of mildew or mold in your grow, good or bad. The good type should be hiding in the media not munching on top of it. While some things like slime molds are harmless, they are just an indicator of the perfect environment for other, more harmful ones.
Keep media drier, a little warmer, more air flow, water more often but much less amount than currently, and reduce your relative humidity.