Keep people and pets away and avoid disturbing the cluster. It may be a temporary swarm resting while scouts locate a new home. If its location creates a problem, contact an experienced beekeeper or bee-removal professional. A swarm being less defensive than a hive does not make handling it safe.
Leaving honey, wax, and dead bees inside can cause odors, leakage, and other problems even after live activity stops. Arrange experienced removal that addresses the colony, comb, cleanup, and building repair. Killing visible bees or simply sealing an active entrance is not a complete plan for a wall colony.