And I guess I just answered my own question by, of course, finding a similar thread immediately after posting this one. Isn't that always how it works after you've spent hours on a topic?
I made a copy of the measure "Semesters to Graduation" and used the following pseudo-code...
IF [Student Majors]="Science" then NULL else [Semesters to Graduation] end
I hope this is helpful to everybody else trying to solve similar problems.
Maybe if somebody has an alternative solution, that would help me mentally sort out how Tableau works!