A person walks past in a face mask and your dog barks, though they ignored the last passerby. The covered face may make someone harder to read, or another detail may be involved. You do not need to decide which in the moment. Give the person room and guide your dog away. Do not ask a stranger to remove a mask or come closer for your dog's training.
If your dog has lunged, bitten, or become broadly fearful of people, talk with your veterinarian and a qualified, reward-based behavior professional. A new response can be influenced by pain or other health changes. Keep greetings optional and avoid narrow places where a masked person and dog must pass shoulder to shoulder.
Build comfort with a helper
Ask a familiar adult to wear a mask at a distance where your dog notices them but stays relaxed. The person should not approach or reach. Reward your dog for looking and then turning back to you. Leave after a few easy moments. On later days, vary the distance or the mask, but not both at once. If your dog freezes, refuses food, or barks, make the next setup easier.
Some masks look and sound different from others. A single successful session does not mean your dog should meet everyone who wears one. The goal is passing people safely and comfortably, with no social obligation to say hello. For more on helping a dog feel safer around unfamiliar people, see the reactivity around people guide, never recruit a stranger to approach for practice
Sources: VCA Animal Hospitals: Fears and Phobias in Dogs - Animals and People. Dogs Trust: Reactive dogs - why it happens and how to help.