Were they ‘stained’ before the dish was installed? The pattern is too random to be applicable to zinc or copper mold kill.
I don’t know for sure, Darkthirty. It’s a bit puzzling. It looks like streaking white stains but not concentrated to a specific area - much more broad, yet isolated to only that part (albeit, quite a large part) of the roof.
Lime stains from tuckpointing might cause that. Seems there is some marks by the other chimney also. Coincidence or correlated?
Looks like Lime from the Chimney has stained the tile to me. You say you have a satellite mounted on that side. This can happen if they fail to seal the holes properly. Could have also been tuckpointed before as Darkthirty said.
That is my suspicion, Darkthirty. Looks like lime-staining to me and yes there is some on the other chimneys a little. I wonder if this is something that might need treated / sealed up.
Hi Dtris, thanks for your suggestion. Is the Lime from inside the chimney or from the fixings around it do you think?
How long have you owned the house? Has anyone repaired the chimney? Have you changed furnaces/flue liners? When did the white appear?
Usually is is from mortar washing out ime. I would get some binoculars and get a good look and see if the mortar looks cracked, missing, or large openings especially where the satellite is mounted.
You can always get a chimney inspection, they assess the outside and the inside of the chimney usually.
Doesn’t appear to be a passive leach pattern.
Dear all,
Thank you for the responses and insight.
I have been informed in good faith by the original house owner that these stains are in fact bird poo. So, I’d like to recognise Ivoman’s prescient suggestion above 
.
Regards
I know my shit! Hehe