Blue legs (Clibanarius tricolor). Just buy a bag of shells from hobby lobby for like $4. Rinse them in RO/DI water, then soak them in saltwater for like 20 min and dump them in the back behind the rocks. That’s what I did and I’ve had the same 9 blue legs in my nano for 4 years since I set it up. With proper sourcing and care, they can live for 20-40 years. It’s crazy to me that they are part of the “throw away” clean up crew and generally purchased yearly. They are a long lived species, you have to take extra care to ensure their survival. I care and feed mine a mixed diet like I treat my fish and corals, and I ensure they have shells adequate in size for them to move into. If you can verify they are sourced from the Caribbean they generally fair well in the aquarium.