B- in a Major Class Last 60 Credits (Mathematics)
Any advice?


Yes.  Since you have a designated adviser, I recommend talking to them about it.  They can guide you through the rest of your program.  You can talk to them when classes resume, which is fine, because you have  long term considerations in which absolutely no action is needed or helpful over the semester break.

Your question here is not really practical or answerable.  Yes, it is better to get higher grades than lower ones: you heard it here first.  Obviously if you could retroactively increase your B- grade, then your hypothetical PhD application would be at least slightly stronger.  No one B- is going to make or break a PhD application.  If you are having time management issues, then you should take steps to address them...starting with talking to your adviser.  You might consider delaying taking the sequel to the course you got a B- in to shore up your knowledge, acquire better time management skills, or both.  On the other hand you might need to take this course right away in the context of your program as a whole.

Finally, I notice that you seem to have more success with biology than mathematics: compare a likely first-authored publication with a B- grade in an intermediate level course.  If you apply to a program in mathematics, then your math skills will be compared against others whose interests are solely or primarily in math.  If you apply to a program in biology then your math skills will likely be found to be very strong and set you apart from many of the other candidates.

Enjoy the holiday break.