After awhile there will be stuff growing everywhere. Coralline algae is mostly what is growing on the overflow in that photo as well as a darker colored algae. To encourage the growth of coralline algae, keep your calcium, alkalinity and magnesium levels in check. Calcium should be about 350-400ppm, alk 7.4-9dkH and magnesium 1200-1350.
Other things you may find in your overflow or sump may include, sponges, copepods, pineapple spongesand feather dusters (probably the barnacles you were talking about). If you google search these you can find photos for each
Be patient, they will come
