I'm using a thing called microMath+ on Android (available on F-Droid) but it's kind of clumsy. I haven't tried hacking on it yet to improve the UI, but I'm wondering if there might be something else out there that's already better. (Not proprietary software, obviously.)
I don’t see the app on F-Droid – the listing on https://www.f-droid.org/en/packages/32/index.html skips from “Microchip” to “Microphone”. I can’t find the app’s website or code by searching the web either – all I found was this C++ project from 2012: https://github.com/candycode/micromath.
Sorry, it's spelled with a Greek lowercase mu.
It wasn’t clear what part of “microMath+” you meant should be replaced by μ, but I eventually found the app. It’s μMath+, on F-Droid at https://www.f-droid.org/en/packages/com.mkulesh.micromath.pl... and on GitHub at https://github.com/mkulesh/microMathematics.