It is an open access initiative. Roughly speaking, green publisher to some extent supports authors' self-archiving of papers published with this publisher. See here for details:
Green publishing is an innovative way to manage publications according to sustainable requirements. Environmental and social criteria are introduced in the publishing process in order to reduce impacts produced during all the phases. and the adoption of the best available techniques and the best environmental practices, taking into account the complete life‐cycle of a product: from design to production, distribution, use and end of life.