Hi there,
If the moss has grown thick scrape it of with a paint scraper. Then being very carful spray bleach directly on to the moss,
you made need to do it a couple of times but it dose work. Liquid pool chlorine will do the same job.
When its dry hose down. But be carful if you have garden around use lots of water. If your porch is prone to moss some time in the furture you will need to do it again.
But is dose last a long time before you need to repeat it.