I am trying to plan for my mom who will be traveling from Guam to the US, but she has a connecting flight in Japan, which has a 2-5 hour layover. She has an expired Chinese passport due to the pandemic, but she has a valid US green card. Would she run into any issues at Japan? I'm assuming mainland US should be okay with letting her in, because she's a permanent resident. Please provide input if you have experience or knowledge in this area. Thanks. I just want to make sure she doesn't get stuck in Japan.
She does not have a passport, so it's possible that she will not be able to board the flight to Japan.
A "domestic" flight from Guam to Mainland USA might be possible, but what you're describing is technically an "international" flight to Japan followed by another international flight to USA. If nothing goes wrong, you probably won't have to cross Japanese customs/immigration checks, but what if the flight is delayed and she has to stay overnight? This has happened to me several times, for example in 2016 when a delayed flight to UK resulted in me having to stay at an airline-supplied hotel in UK, which meant I had to go through immigration and customs in UK and get a UK stamp in my passport: it is not uncommon for people to be prohibited from staying in the airport for 24 hours, and in fact some airports in Japan such as Osaka Itami are not open 24 hours/day (you haven't said yet which airport in Japan this will be). If the flight is delayed and she misses the connection, she will not be allowed into Japan without a passport, and will not even be allowed back to China with her expired Chinese passport, so she will be sent back to Guam where she might not have the easiest experience (due again to not having a passport, even though she has proof that she's allowed to live in USA).
I would recommend that she first applies for an "emergency travel document" or a renewed Chinese passport before booking any flights.