This is probably L.S. residue.. is this a new tank? can you get some out with a plastic cup or BRNE SHRIMP net. do you have any surface agitation? and what do you have for filtration? your skimmer and pre-filter box will help immensely.
Is anything in the tank yet? If not, then wait a day and it should be cleared up. Be sure you run your skimmer. It might be proteins. Cycling tanks tend to behave rather oddly. I wouldn't get to worried.