Worm is still OK if you have difficult access to fruit flies, baby crickets. To be 100% safe, you can also pre-kill the worm before feeding your sling. Your sling of that size will typically molt about once a month. Just keep a record. From the photo, your substrate is way too wet. You can throw in a small water dish or just wet a small portion of the substrate. NOT wet the whole substrate. Suggest you change your substrate or rehouse into dry substrate.