How to get rid of mold on a painted I-beam on a porch?

Vinegar will kill mold spores. Then you need to spray with a hold inhibitor for the future. I use wet & forget outdoor but I'm not sure how well it will work in a porch.

Bleach and vinegar can both kill mold, but vinegar is much more effective for removing mold from porous materials. This is because bleach only kills moldspores on the surface of affected materials. Vinegar will penetrate porous materials and kill the mold at the roots.

Clean it up with bleach water to try to minimize stains. Spray on straight vinegar to kill the mold spores once you rinse the bleach water off and let it dry for a bit. Let it dry naturally to kill as much as possible. If it stays damp from humidity in the warm season, reapply the vinegar to help keep the mold/mildew away as long as possible before it can take hold.

If vinegar doesn’t work, Behr makes a very good mold-killing deck cleaner. You’ll have to scrub it with a long handled brush.