Is that a new project you want to power wash? Often the pressure treated lumber has a greenish cast to it when purchased anyway, that is from the chemicals I suspect. Your wood looks pretty new to have algae growing on it. Or maybe the picture color is off. If you power wash it, be careful of which nozzle you are using. And be careful where you are spraying. We rented one a long time ago and I remember having the just straight spray nozzle on and when it accidently sprayed across my bare foot it actually left a slightly bloody streak. So also if you are real close to the wood and the spray is a tight stream, it can actually almost cut the surface. We now own a power washer and I can adjust the type of stream. It isn't the most powerful machine but we have used it several times. In fact I power washed the liner of my little pond to remove the algae a while back.
