It is the same quality as you get in the EU. Milk lasts a week or so in the US as well. UHT is mostly unheard of in the US because people actually like the taste of milk... ;)

Someone who is buying real cheese in the US is not really in the wheelhouse for soylent, we are probably talking about a big block of mass-produced yellow cheddar here. Most likely it is being stored improperly in a ziploc bag or similar and the retained moisture is causing premature mold growth.