The term "IP address geolocation" is common and descriptive enough, and isn't trademarked. The mark "GeoIP" is MaxMind's trademark for their particular IP geolocation service, and isn't something you can use just because you're familiar with the term, regardless of if the application is free/open source/whatever.
While you likely mean well, MaxMind has a legal and practical obligation to defend their mark, which means issuing C&Ds for people who are using it without a license to do so.
