Generative AI threatens to disrupt how we work, learn, communicate, and participate in online communities. We performed a qualitative interview study to understand how online communities on the social sharing site Reddit are challenged by AI-generated content (AIGC) and how they are adapting. We conducted fifteen in-depth, semi-structured interviews with subreddit moderators about their experiences moderating AIGC. Though our participants see both legitimate and illegitimate motivations for using AIGC, on the whole they view it as detrimental to their communities, with a level of concern that is dependent on the purpose and size of their subreddits. Moderators reported developing rules and using a variety of strategies that may help communities prevent or curb AIGC, but without foolproof detection tools, enforcement is challenging and relies on heuristics. Overall, for online communities, the threat of Generative AI is not speculative: the disruption has already begun.