I agree it's probably efflorescence and seems to be coming from your mortar joints. If you clean it off (can use a stiff bristled brush and water or surekleen and an acid brush), seal your coping and mortar joints with a silane/siloxane sealant and the problem might not return. Seal your deck while you are at it. The degradation may be due to salt damage as the water evaporates, but also if water is in the concrete and heats up, it will expand and can damage it as well.