It could be a protein film. Does it form on the top of the tank as well? If it doesn't it could be just that there is sufficient current to keep it from forming. A protein film is perfectly safe for fish, although if it is on the top of the water in the tank and you have labrynth fish it may cause a problem. In the filter shouldn't be a problem if it is indeed a protein film.
Just as a side note, your corys, tetras and hatchets would prefer to be in larger groups. And as you have plenty of space that is what I would do IMO.