I've seen the same thing in my tanks, but not to that extent. I believe it is organic waste (e.g. protein) from leftover food, fish excretion, etc. How long has this tank been up and running? Any recent changes, additions? If this occurs, when doing a water change (before siphoning) I'll skI'm the surface of the water using a plastic cup (hold the cup sideways and quickly dip it in to suck the matter inside the cup, dump out, and repeat). I also use surface skimmers on my tanks, so I no longer have any issues.
How much are you feeding? What type/brand of food?