Americans, or citizens of the United States of America, are visa-exempt for Mozambique, but this does mean that they enter freely.
Nationals from visa-exempt countries must still:
Carry a valid American passport that still has at least 6 months validity
Pay 650mtn (about $10USD) on arrival (take care of the receipt, you'll need it throughout your stay)
Show proof of accommodation reservations (paper copy)
Show proof of onward travel (this could be a plane ticket leaving Mozambique, or the next country you are travelling to if leaving Mozambique via a land border. A bus ticket is also valid)
Show they have means of subsistence (paper copy of bank extract)
Items 2 & 3 are highly recommended. Item 4 is not always required however it is best to have it to avoid potential hassles.
After paying 650mtn and stamping into Mozambique American citizens can stay up to 30 days and they can leave and return as many times as they wish during these 30 days.