That's probably just because whoever originally added the artist had an easier time typing the micro sign (seems like that's the one on Mac keyboards) than the mu, or just copied the wrong one. From the other threads on Unicode -- especially when concerning the ʻokina -- we do prefer the correct symbol when possible. In this case, the pun does strongly suggest that mu is the correct character to use. I don't personally see any reason you wouldn't correct this, and this is one of those cases where you'd probably want to make sure all of the "rename artist credits" boxes at the bottom of the page. Don't forget the edit description either, given how hard it would be to see the difference in the summary.
EDIT: As for the formal rules, it's pretty much "use Unicode if applicable" and "update if the name is wrong or the artist is better known by something else (as long as they aren't separate projects), but don't change the artist credits unless you have a good reason to." This is a good reason.
By the way, the alias already seems to use the micro symbol, so that probably won't have to be changed.