你可以給hiring manager (and/or team leader)發個email,

一般interview完不是都要發Email道謝的嗎，你正好利用這個機會再順便assure them that you won't have visa problems.

The email can go something like this:

I wanted to thank you again for taking the time to interview with me about this position. ...blah blah....

I would also like to take this opportunity to further clarify my visa status. .... (如果你有幫你辦綠卡的移民律師，你可以把他/她的聯係電話附上，說明in case they have questions please contact this lawyer.)

Email 最好簡潔明了，不要太長（尤其不要解釋很多細節)，just make your point.

Good luck -- hope you get that offer.