From what I can tell, it looks like it is largely particulate from the sand and rocks. You may have to deal with this for a short while, until the dust settles (so to speak). However, there will always be a residue to deal with at the water line. With salt water tanks, the surface tension is higher than fresh water, so this naturally attracts dissolved organics. Essentially, the surface of your water will skim minute amounts of protein and other dissolved organic compounds.
That said, if you have a decent magnetic cleaner (one with a metal or plastic hard edge), it only takes a few seconds to clean up...