I said if it looks oily, it probably is. It could be some particulate matter that was on the base rock, or the sand. If it is not oily, and is simply on the surface hangin' out, try to slowly scoop it off the top, by barely submerging the lip of (preferrably a wide) a new/never-used container. This will "skim" it off the top. If it layers onto itself when you blow on it, this is what stagnant water I have witnessed does. It kinda skins over. Do you have surface agitation?
125 gallon saltwater: 55gal sump, 4 Blue-Green Chromis, Purple Tang, True Perc, Firefish, Royal Gramma, 5 Ricordea, Bubble Coral, 15 Pulsing Xenia, Green Star Polyps, Deresa clam, Green-Tip Torch Coral (about 11 "heads"!), Orange Montipora Cap, Purple M. Digitata, Green Slimer Acro. Yongei, 3 Orange M. Digitata, Pink&Green Acro. Millepora