I won't use a ladder on any decking that is wet or damp regardless. Same goes for algae covered footpaths and surfaces etc.
Having said that, the only fall I've ever had window cleaning was on wet decking. I wasn't up the ladder, I was carrying it back to the van. I put one foot on the decking and was like ice.
Fortunately, no bones broken - just some extremely painfull bruises. One of which was to that funny bone bit at the base of the spine. Sitting down was agony for a week afterwards.
A mate who I sometimes work with also hurt himself on wet decking. He uses a ladder mate and to prove to me it was 100% safe even on wet slippery decking, he took a short run up at it with the intention to jump and land on it with both feet to show it wouldn't move.
Unfortunately, while taking his short run up, he slipped and missed the laddermate, came a real cropper and landed on top of it instead. I almost wet myself laughing.