Grades in a PhD program
I'm an advanced graduate student in Physics, and I've gotten a B- in a class. I have also gone through rigorous job application processes, and generally, nobody will care about your GPA.

In Academia, papers, references, and networking will determine your postdoc or faculty position, grades won't even be considered.

If you plan on going into industry, a completed PhD and the brand name of your institution might be very important to get an interview, and depending on the job your comfort with problem solving and advanced technical concepts might matter. Companies will generally only look at your transcript after they've hired you to confirm your enrollment and completion of the program.

One B- is nothing to worry about, especially for a PhD student!