Repaint moldy porch chairs
When I've had to re-stain decks or chairs etc with a little mold on them I've always used a bleach solution.  Mix a little bleach with some water and apply it to the mold (spray bottle, or brush).  The bleach will kill the mold quickly (just let it sit there).  Then lightly sand the mold off (just to get rid of the moldy texture) and re-paint.

Good luck