Film on top of Sump Water

Hi,



I have a strange, clear film on top of the sump water but only in the return pump compartment. Sump has three sections for inlet fliter/skimmer, then substrate middle section with cleaning algea, then the return pump section (last).



Only the last return pump section has this film I think because the water is more calm here. Is this normal? DT is fine...?



Thanks!