"Mueller" is of course Jeremy's best friend who was regularly seen on Little People, Big World.
The most recent thing involving Mueller that drew some attention was a video last summer that featured Mueller, another Roloff friend named Christian and two girls from France drinking Jack Daniels in the Roloffs garage.
If you're unfamiliar with Mueller, you can read our piece on Mueller from last year:
Mueller has been involved in nearly every activity/event that has resulted in controversy with Jeremy (cruel fan insults, many of the slur comments, the drinking stories and the cat tossing, etc). Matt and Amy have stated that they consider Mueller to be a "great" person and are happy their kids are such good friends with him. Jacob Roloff is said to idolize Mueller. The amount of time Mueller and Jeremy spend together has generated discussion over the years as Mueller often accompanies Jeremy on Roloff family trips.
For the record, we are told by friends that Jeremy and Mueller feel that they are unfairly scrutinized simply because other people don't understand the depth of their friendship (aka. people are jealous that they don't have a best friend that they are as close to as Jeremy and Mueller are to each other).