Mold can be one of the more challenging issues to deal with as a homeowner or renter. It can be nearly undetectable and can cause health issues, both short term and in the long run.
The stuff can get into attics, basements, crawl spaces, and even walls. It is often accompanied by musty smells and can cause asthma-like symptoms in the short term. As a renter, you are within your rights to contact the landlord and report it and they are required to have it investigated. This is not something to be taken lightly and you ought not do so. Deal with it aggressively and get rid of it.
Mold to a certain level is always in the air and that is fine. You just need to be sure the level in your house is within acceptable standards. A site with lots of information is Restoration Master Finder. There is a lot of information here that can be helpful to you to learn about the different types of mold as well as techniques for getting rid of it and contractors you might use who can deal with it.