If its just cleaning the algae off, then as said all you need to do is wet it and then spray on the shop bought bleach as mentioned.
Looking at the pic, the run off will go on pebbles and im taking it, that the decking is not flush against the wall, so shouldnt fall on the decking.
However, if your doing a spring clean of the decking and slabs as well, then invest in hypo. So jet the slabs first and then apply at 5% to the slabs . The decking, im taking it your using an electric pressure washer, so use a 45degreee / 25 tip to strip the decking from a height. Leave to dry and then treat / paint.