Mildew in new apartment
Comfort and mold are two different things.  The comments you received are good advice for comfort, but mold has its own considerations, especially if you live in a hot, humid area where interior mold is a prevalent problem.

55% relative humidity is the maximum to avoid mildew; 50% is usually the recommended level.  Dehumidifiers are inefficient below about 40%, so you'll spend a lot on electricity without pulling much moisture from the air if the humidity is at that level. 

Moisture gets absorbed into every porous surface, so if you start with enough moisture to promote mold, you will need to run the dehumidifier for days to dry things out. 40-45% can help initially, but once you get things under control, run it at 50%. It will cycle as needed to keep the humidity below the mildew threshold. Over 60% you'll get new mold blooms. 

As others have indicated, where you live can be important to knowing whether you have a bigger problem.  If you live in a place like Florida, interior mold comes with the territory and you just need to deal with it.  Mildew caused by environmental humidity will bloom in a widespread pattern.  

If you live in a cooler, drier climate, it would be unusual for interior humidity to get that high on its own, so interior mold could be a sign of a water problem (like a leaky water pipe or rain or ground water getting in somewhere).  If you get mold in that situation, it is likely to be concentrated in the problem location.

There's tons of information and advice online.  Here's a link that covers a lot of information in one place, and it's written for a non-technical audience: https://inspectapedia.com/sickhouse/Humidity_Level_Target.php

BTW, @Fizz makes a good point in a comment.  Drying things out doesn't kill the mold, it just controls it.  After a bloom, there will still be spores everywhere waiting to regrow.  You need to clean and disinfect all surfaces to get rid of it.  Vacuuming with a regular vacuum cleaner will just recycle it back into the air; use a HEPA vacuum cleaner (there are consumer models that aren't outrageously expensive).  

If you live in an area where interior mold is endemic, consider getting a HEPA filter to capture the airborne spores, which will reduce what's available to bloom if conditions become favorable.  There are HEPA-grade filters for your HVAC system, or high-end pleated "allergy-rated" filters.