Collard greens — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Steamed and plain is safe in small amounts.
The full verdict, portion guidance and serving notes are collected on the Collard greens food facts page.
Short answer
Yes, that's perfectly fine. Collard greens are safe for both cats and dogs when offered plain and steamed in small amounts. Since only your cat ate them, there's no need to worry about your dog. Just offer the greens as an occasional treat, and call your own vet if your cat shows any unusual signs like vomiting or diarrhea.