You can add to your crab's diet by providing it with fruit or vegetables, like kale, broccoli, and bananas. For a longer-term option, give your crab a bit of wood, which hermit crabs like to play on as well as eat. Hermit crab shells by the opening size.
In the wild they are omnivores, feeding on both plant and animal material. This is an Alaskan Hermit Crab (who resides in front of Port McNeill, not Alaska). Living on his/her back is a "Hermit Crab Sponge".