What's this white stuff?

I bid a 2nd level deck (cedar 2x4's for decking, about 10 years old, never before cleaned or sealed) and there is this weird bright white stuff on the bottom underside of the deck that the client wants removed. They already attempted to clean the deck themselves and used an off the shelf liquid deck cleaner with sodium hydroxide and sodium hypochlorite in it. My first thought was that it didn't get rinsed off very well and dried white on the bottom. But, they said they had a some of this white stuff on there before cleaning, just not as much. Could I be right, or is it some kind of mildew? I splashed some water on it and it didn't instantly emulsify but you can scrub or scrape it off when wet, not sure if it'll come back again. Anybody see this before?

It will wash right off

The fact that it is over top of existing algae or green mold in the picture would lead me to chemical residue based on what you describe. When you redo this deck make sure you have some detergent to give it some cling so that it is effectively removed.