A reasonable and benign explanation is that Trump wanted Mueller's input and suggestions about WHO would be good or best to head the FBI. A consultation, not an interview. Mueller knows many of the top players in FBI/DOJ, past and present.
The fact that the two met does not mean Trump necessarily was interviewing Mueller to head the FBI.
I think Trump was sizing him up for his role as SC, but I am not convinced that role was disclosed to Mueller at that meeting.