If there isn't a delete option, try disconnecting your phone from the system, which can sometimes clear stored messages. And also check the manual for specific instructions for your Mokka’s model.
Same problem here! Got the bonnet resprayed last year, and now the windscreen pillars are acting up. To make things worse, the passenger door by the window edge is starting to show signs too. Ours is a blue 2016, by the way!