Protecting the environment is an important topic. In this study, college Chinese as a Foreign Language (CFL) students engaged in an agentive process of informally learning the Chinese language and culture by watching "The Mermaid" a movie produced in China in 2016, which focuses on sustainability. Informal learning is recognized as one of the most powerful factors in Foreign Language (FL) development (Furlong and Davies 2012). Limited research on informal CFL learning from watching online videos is available (Huang and Lammers 2018). By using key ideas of agency, mediation, and internalization in SocioCultural Theory (SCT), this qualitative case study found that informal video learning from watching "The Mermaid" was helpful for learning vocabulary, grammar, and cultural values in sustainability topics. By means of semi-structured interviews, observations, recalls, and documents as data, the findings suggest that using real-world movies such as "The Mermaid" as an environmental sustainability topic had a positive impact on FL education. Demonstrating their agencies, learners engaged in vocabulary, grammar, and cultural value learning. This study offers pedagogical implications for language learning in Higher Education. Language Centers can therefore play a role in engaging campuses in social topics such as environmental protection.