It’s very common and is not usually considered a good or bad thing. Sometimes professors have reason to travel to other universities because of research in which they are involved – or other compelling reasons.

Sometimes universities will actively recruit a professor who may be considered visiting before a new job is posted. i.e. he is getting the lay of the land for a position where someone is retiring – or a new field is opening up.

Most universities and colleges will have visiting instructors, who are perhaps waiting for permanent placement elsewhere but are totally qualified to teach their subject. Oftentimes a PhD candidate [with 2 college degrees already achieved, waiting for the icing on the cake] will take that kind of job, sometimes a visiting instructor has some special background or skill that an academic department needs to fill in a class or two… It’s a good situation, IMHO… they’re not at ALL like a “substitute teacher” back in high school.. they’re a qualified temporary position every bit as good as permanent staff in a college department. I once had a very famous novelist as a visiting instructor in an English class… they were an inspiration to us all–even though teaching wasn’t their main gig, so to speak.

It’s fine. Non need to worry as long as you’re learning and getting something out of the class. Lots of top universities have visiting faculty members, many of which are reputable and respected in their fields.