Does the porch roof get much sun, if any at all? Do tree branches hang over the porch roof, or the roof above, where the mold is growing? If you have gutters on the eaves above, do you know if they're 'free and clear' of all debris?
In order for mold/mildew to grow, there need to be certain 'environmental conditions' to exist. There needs to be a certain range of temperature and a source of moisture. If the temp is warm enough, and there's limited or non-existant sun on the roof, along with dampness or dripping water, you've essentially created optimal conditions for the problem to exist. The best way to correct it is to trim back your trees (to allow sun in and to improve air flow), and to inspect and clean your gutters (above) and downspouts. During the next heavy rainstorm, put on your boots and raincoat and grab an umbrella, and walk around the ENTIRE house to observe how the water is running off your roof. Also, it's possible you have LEAKING gutters too. They'll need to be sealed with Gutter Caulk (when cleaned and dry.) Observe if the water is running OVER (or behind) your gutters and have that fixed. Also, observe where the water is running to once it leaves the downspout. You want it at least 3' from the foundation and you want the water to run AWAY from the house; NOT back towards it.
Correcting these 'problems' will require that you climb a ladder and possibly get on the roof. If you're not comfortable w/heights and such, hire a Pro (or get family to help.) Really, this is a very easy job to do. Again, in short, 'open up the tree canopy' (to improve air and sunlight), and fix any 'leaks' in your gutters, or install them where they should be.