Short answer
A whole crab leg is too much crab for a cat. While the cooked meat itself isn't toxic, that amount can cause stomach upset like vomiting or diarrhea. The main risk is the sharp shell, which can cause serious internal injuries. Offer your cat a bland diet and call your own vet if you see any signs of discomfort or trouble passing stool.
Crab — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Cooked crab meat without shell or seasoning is safe. Shell splinters cause internal injuries.
The full verdict, portion guidance and serving notes are collected on the Crab food facts page.