Mine looked like a rainbow so,.. Lol
Could be a number of things of witch is out of my skill level, I would however try and remove it,
Get a paper towel, (scent and chemical free) rest on top of scum and quickly remove,
And add more surfice ripples, I'll do some research and see what I come up with
Could be brown algae, could be oils from your skin or the air in your house,