So there is nothing but one hermit crab in the water that you're trying to skim?? If you only have a hermit crab in the QT, how do you expect the skimmer to be pulling anything out? Skimmers pull out organic material (fish pee and poop, excess food, etc). If you don't have any livestock putting out organic material, there won't be anything for the skimmer to collect.
Put that skimmer on your main tank and see if it pulls out anything. Another possibility is that the skimmer may need time to "break in". I experienced this with my ASM skimmer.