What is the white stuff I’ve seen on some of the fence walls? How would I get rid of it?

This is an indication there has been some kind of water damage. Concrete block walls traditionally are built because they are strong and long-lasting. If the walls have been built properly, a water / moisture seal, commonly known as waterproofing, has been used and there has been no unusual strain or damage to the wall, they can last a very long time.

Concrete block walls will soak up excess water into its pores and will cause this ‘white stuff’, or in some cases mold growth. One of the first things you will need to do is assess the water usage along the wall. If you are overwatering and the ground appears to be saturated, the wall can soak up the water. Cut back on the watering or remove the plant materials that are too close to the wall. If the excess water is not controlled, eventually the wall can break down causing structural damage that will cost more money to rebuild. For repair I recommend you get a professional opinion from a licensed contractor in this field.