@Marnus Why do you need MMM-alexa if you have AlexaPi up and running??

You may want this https://github.com/dgonano/MMM-AlexaPi (but it is not that necessary just a way to know if you are actually talking to the mirror and that you know it is working on that point in time) and even for that to work you have to have declared magicmirror as a device type at the beginning of the AlexaPi installation, however it can be changed.
Note: When installing AlexaPi be sure to set the device to magicmirror and not raspberrypi (or other device)

Good luck

@marnus There are a couple of Alexa integration modules. A quick search through the module showcase section of the forums will give you some threads about installing and configuring these modules.