This customer's HOG1.3 scrubber® in freshwater shows how the Green Grabber® rocky surfaces on the back panel hold on to loose slimey growth. This is a good example of how freshwater growth is different, and needs different attachment surfaces. Although this type of thin non-hair growth can occur in saltwater, it's much more common in freshwater and does not hold on well to screens, or strings, and this is why the HOG1.3 (and HOG.5 and 1 and 1x and 2 and 2x) do not have screens or strings.
The HOG1.3 can also be used in saltwater, however.