W-2 Part time employee getting paid while Non-Permanent Resident in Japan

Hello, I was searching for some guidance on tax implications. I am a US citizen living in Japan as a non-permenant resident <5 years. I recently have started working part time from small business employee in USA and recieved my first paycheck. I am a W-2 employee and state taxes + social security are witheld from the paycheck, but not federal tax because it is under the threshold. <1000 usd/month. I would appreciate any guidance about how I should file taxes here in Japan and what I should anticipate. Also, I would like to protect my employer (who is family) as they may not be aware of tax implications they may have to pay. So I would like to relay them any prudent information. Thank you.