I am an American citizen, do I need a visa to travel from Hong Kong to Macao?
No you do not need a visa to travel from Hong Kong to Macau/o. As both are what I call civilised countries which stamp most people in on the borders, TCG had a shit load of trouble last year getting visas for each country and having 3 visas running at the same time. The only reason you'd need a visa is if you are travelling overland into Macau say via the cheapo buses which travel overland via Zhuhai. Note the Lo Wu visa $80 visas are no longer issued to Americans and it can take 4-5 days to get it processed in Wanchai doing it the proper way. Just take the turbo jet (hydrofoils) to Macau and no visa is needed. You will however be stamped in and out and back in again on your return to HK. However HK and Macau immigration officers are not arses they will pick a discreet corner to stamp you in and out. The Taiwanese immigration officers always stamp right in the middle of pages and get ink everywhere taking up a complete page.
This also serves as damning evidence though as Macau is.... how shall we say filled with adult entertainment (gambling, and overt WGs) therefore you may need a visa of a different kind.