bad because of..
first this film will reduce/block the light that goes into your plants.
second it is not strictly do harm to your tanks, but it is aggregation of bacteria, and not a welcome guest.
removal, use the plastic wrap(saran wrap) toss it on the top, and it will stick to it, then trash. use a piece of paper towel, or newspaper.
best way is to create a wave on the surface of the water, then it will not form again.