Turns out it's not uncommon. A Google search reveals that "Alma Matters" is used in many alumni magazines, including places like the University of Minnesota, SUNY Potsdam and Tufts. As you say, it's not a hard pun to come up with, so Morrissey probably thought of it on his own. However, his use of the pun is the most widespread, dominating the Google results for about 20 pages. So I guess he "owns" it.
Originally Posted by Not Right in the Head
Incidentally, I discovered that "Alma Matters" could have another, heretofore unexplored meaning:
"ALMA is the Atacama Large Millimetre Array. ALMA will be the largest observatory ever built operating at millimetre and submillimetre wavelengths in the world, and will provide British astronomers with dramatic new insights into the formation of stars and galaxies."