Short answer
This is a low-risk situation. A single lick of the white membrane is very unlikely to cause harm, as the main risk comes from ingesting larger amounts of the rind or membrane. Just watch your dog for any signs of stomach upset like vomiting or diarrhea. If you see those, give your regular vet a call.
Pomegranate — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
A few arils are fine, but the rind and inner membrane are high in tannins and can cause vomiting.
The full verdict, portion guidance and serving notes are collected on the Pomegranate food facts page.