It's not mould. It's green algae and you can find it on many cars where water gets trapped in confined spaces like between the rim of the lights and the bodywork, in the bonnet abd boot shut lines, areas of the grille, etc. Seems more rife on light coloured cars. My wife's white MX5 was a devil for it and the silver Polo wasn't much better, but the black MX5 never suffers from it and the red one only very slightly.
I kill it off with anti-bacterial spray but test it on your paintwork discretely somewhere. It just required a twice yearly treatment on the white MX5. Prior to that I was trying to clear it every 2-3 weeks.
It's a product of infrequent washing and poor drying/airing of the car and is more prevalent during damp spells - autumn/winter, rain, fog and mist.
