This has a big flaw. If the future visitors copy all passwords to a usb stick and take it with them then anyone in the present time that gets a copy can pretend to be from the future. Worse still, everyone would believe the phoney future visitors without question.
Yes but on that case the earlier suggestion of just asking them for a stock market prediction would be good enough without implementing the password system.
But then you have to wait for the stock market prediction to come true. Instead, since you already know your temporal password for the year, the proof would be immediately available.
I'm pretty sure no-one from the future will have a way to plug his/hers mini-USB-XC or whatever the usb name will be at the time in the future (USB standards are just a mess).