What could be the possible reason(s) for building up of mould / discoloration of the roof tiles near the chimney? What are the DIY / expert solutions to fix this? I am attaching an image of the roof near the chimney area. The house is a semi detached and on the other side of the chimney (neighbour's house) there is no visible mould / discoloration.
That patch actually looks cleaner than the rest, to me, & I see no mould, only moss. They say moss only grows on North faces, but that's not entirely true. It's perhaps remaining wetter for longer than the surrounding area; possibly shaded or in the lee of the prevailing wind.
Check if the rest of the estate shows a similar pattern.
If you're going up to have a proper look, check this flashing too -
It's hard to tell from this angle but it looks one course short.
