Actually its better to have the scum floating at the top. That's the theory of a Calfo skimmer. The trick is just finding a way to remove it. That's what I don't like about the HOB skimmers. They pull the water from the bottom instead of the top where the waste accumulates. With a surface skimmer you don't want any agitation of the water on the surface. You're letting the lower density of the waste work to your advantage to accumulate it on the surface.
Since you are pulling from below the surface it may be better to stir it up with a power head so your filtration can get to it.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano