Outdoor newly rendered wall showing signs rustings on the edges

Hi there As part of landscaping project, we had new raised plant beds built using grey blocks which were rendered using sand and cement with appropriate additives. We painted it once rendering completed.The edges of these beds where beads were put are now showing sign of rustings that are gradually appearing and growing through the whole length of the beads. The landscaper says it may be paint reacting and we can use stain blocker paint to prevent. Can you advise probable cause and potential remediations for the same please?