How should I caulk aluminum capping where it meets brick?
Backer rod and sealant is appropriate for joints between dissimilar materials such as that pictured in the question. Preparing the brick surface, correctly sizing the backer rod, and properly installing the sealant may all play a role in long term performance of the system.

Select the sealant for your application based on manufacturer's literature. The label on the tube often contains clear guidance.

Filling voids behind the joint with fiberglass insulation or with spray foam insulation prior to sealing may also improve thermal performance and reduce air infiltration.