Whenever your dog has this look on his face, stop and take a break. It usually means the dog is overwhelmed.
Whether entering a new building, meeting new people, or greeting strange animals, dogs become overwhelmed with their surroundings. Just go slowly. Stop to look at the outside of the building. Go to just the entrance of the door, stop, look around. Walk inside, stop, admire the architecture.
New surroundings, people, animals, or anything uncommon to the dog creates stress. Do Your best to minimize their jitters by taking the world in as they would, slowly.