Is this a new set up? It could be boyuant dust from new crushed coral, or if it looks like an oily film it is most likely DOC
(dissolved organic compound. If it's dust, you should be able to skim it off by putting a cup barely below the surface so it pulls the surface water, dump it and do it again. If it's DOC
, the same can be done, but it is a stop gap measure, cause it's gonna come back. For doc
, you can redirect it with good water flow, but it will still be there. The only permanant solution for the DOC
film is a surface skimming overflow box to a sump. This will pull it from the surface and mix it with the water so it can be pulled out by the skimmer.