I have used fsc but with varying degrees of success. Mostly down to the length of time the decking has been left to turn green! the algae staining appears to be in the wood not on it, so it doesnt always come clean.
I use between 1/2 and 3/4 pressure as some decking will splinter or mark with full pressure.
I dont come accross decking very often myself but others on here seem to do more of a restoration type service so will be better placed to give you the correct process for completing the job.
steve