Looks to me like the remains of a previous coating after it has broken down then hit with high pressure , it has then aged some more and has become covered with algae. It doesn’t look like overspray from spray painting siding.
What can cause grid effect is overlaps when roof was sprayed .
The paint used on roof may not have been rated for exterior use that is why it has broken down to that extent, some scammers use interior paint/whatever they can get their hands on.
Makes sense. I’m avoiding it. No sense in trying things out again there. Just was looking for an answer/explanation for the customer. Thanks Aussie!