A mossy porch problem can be killed with a vegetation killer by ROUNDUP, FINALE, or TRIOX. There is a product just for moss and algae called SAFER MOSS and ALGAE KILLER. The nice thing about SAFER is it kills the moss and algae but not most area vegetation.
Spray a fine mist on the moss growing on the porch will stop the existing moss. Wait a week for the growth to die. This makes removal easier. Then remove the dead growth with a solution of bleach and T.S.P., (trisodium phosphate, a commercial cleaner sold at hardware stores). Mix about a one-quarter cup of T.S.P. and a one-quarter cup of bleach to a gallon of water and apply to the porch.
I like to use a scrub brush on a stick so I can work standing up. Wait 10 to 15 minutes for the solution to loosen up the crud. Now reapply a light second coat, this time scrubbing a little. The growth should be running for mercy and can be rinsed off with plenty of water. When the solution has been on the porch for 10 minutes it will be neutralized enough so that it won’t hurt any plant life in the surrounding area.
Moss growing on surfaces is a sign of dampness; just know why there is moisture.