This is common and something that you should be able to clean yourself depending upon how far it goes into the A/C duct. This also means you are not getting enough moisture out of your bathroom. If you have a bathroom window, I recommended you crack that open.
Also, black mold is far more common than the other poster suggested. I know from first hand experience having been made extremely ill myself as well as my children for a very small amount of mold in our home due to water damage (a very small roof leak) and a leak in the bathroom. So, you are very right to be concerned. 2/3of the buildings in Texas have water damage. I do agree there are unscrupulous remediation companies that will charge you a fortune, BUT if the remediation is not done correctly it can result in making the problem far worse, due to the toxins released during the remediation. That's what made me so very, very sick. Also, one of the reasons there is so much controversy about this is that how sick you get and whether you are impacted depends upon your genes.
There are actually 3 health problems associated with mold: 1) allergies to the mold itself - the mold in your bathroom can contribute to this problem 2) many molds produce mycotoxins - they damage the brain and immune system - you CAN have mycotoxin producing molds in your home from ANY water leaks that are not properly cleaned in 24 hours - mold LOVES to grow on wallboard/wallpaper and between walls. 3) systemic fungal infections from inhaling the mold spores - aspergillus is a very, very common one that causes this problem - I had aspergillus in the bathroom where we had a water leak.
Sorry to get on my soapbox, but I wanted to make sure everyone has the full information - there are two sides to the mold issue and one of the reasons folks are not so aware is that the insurance companies don't want to pay for the damage to homes NOR for the healthcare costs for those who are made ill. The same law firm that worked for the tobacco companies back in previous decades are now working for the defense in cases related to mold damage and illness. An excellent website on the impact of mold and health is www.biotoxin.info (this is the doctor that helped get myself and family well). There's info on the genes on this website, as well.
So, I don't think you can ever be too conscientious about being concerned about mold in your home or water damage - the concept of small amount does not apply to some people.