Cleaning aluminium
It depends on how bad the tarnish is. As coffee is acidic, generally speaking, you shouldn't have too much trouble unless, of course, the poor thing's been through the dishwasher.
I have a stainless steel Bialetti I bought in Verona about 30 years ago. My aluminium makers I keep shiny with ketchup.
If, however, things are really dreadful, do the same as one does to restore an aluminium pot - lots of videos on Youtube about the process.
Basically, you clean everything with a stainless steel soap pad. Once you're clean, you can buff with an abrasive to remove the oxidation. Smokers toothpaste works really well on the bad stuff, a soft-scrub product for ceramic range tops is a step up from that. Rouge and a buffing wheel up from that, emery and a buffing wheel the absolute furthest I'd go.
But - clean first, try toothpaste. Finish with ketchup. You'll be surprised.