A fluval 204 on a 30 gal
tank is not much in the way of flow. Unless the return from your canister filter is aimed along the surface of the water your most likely seeing a build up of organics. The skimmer cannot remove it from the water as long as it's on the surface. It needs to be constantly broken up and mixed into the water column.
I would definately suggest some additional water flow, Maxi jet 900 perhaps. Be sure at least one pump is constantly rippling the water surface to break up the tension and allow the skimmer to remove the nutrient.
What you are describing as far as brown stuff sounds much like leftover flake food residue?