These Moon cactus are very cute, but don't live for years and years. The top portion cannot live without the bottom. They're grafted together. You could slice the bottom green part until you see healthy flesh. Let callus over for a couple weeks in a shady dry area. Once a scab forms you can plant into fast draining soil. No water and keep in a well lit room. No harsh sun for your rootless plant. Once it gets roots you can water, and start acclimating to more sun. The little larvae are not normal:).

I don't know how much this plant means to you. So I hope I don't offend you by saying this:). To be 100% honest with you I would just throw it out and buy a new one. These don't live for long. You could go through all that work, and it will probably die in a year, two years tops:). Go to Home Depot or Walmart and buy another. Normally this is not the advice I would give, but I'm trying to save you from a headache. It's spring/summer now so Moon cactus should not be difficult too find:). Goodluck:).