I assume that you mean green algae or moss? I would treat it with a strong bleach solution, 1 part bleach to 10 parts water. Just apply it to the fence with a brush or a pressure spray and this should kill the growth. Give it a light sanding to remove the surface dust and then paint it.

I used a pressure washer on mine, came up a treat, waited for it to dry out then I painted it with wood treatment :o)

I use 'Armillatox Outdoor Cleaner' it is soap based and safe to use. Apply when the weather is dry, it takes a week to work. I do it in the evening when no one else is around and the windows are shut as its a bit smelly for a few hours.