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What should you do if you find mold growing around the AC register? What does it mean? How did it get there? Are you going to die? Well, yes, you will probably die eventually, but not from this 
How did it get there? First, you must understand that mold is secondary water damage, which means, there must be moisture or humidity present. So where did the moisture come from? The same place that makes water form on the outside of a cold glass of water, condensation. If you remember in science class, dew point is when the moisture in the air reaches 100% relative humidity. Cold air cannot hold as much moisture as warm air, so when warm, humid air comes in contact with something cold, it reaches dew point and makes condensation, or water.
The air coming out of the air conditioning vents is about 20 degrees colder than the inside air, so if the air inside is too humid, condensation will form on the metal ducts at the opening and at the register. So what can make the air inside too humid? Setting your thermostat to “on” instead of “auto”, turning the thermostat up too high in the summer in a vacant house or seldom used upstairs, turning the thermostat down too low in the summer, having leaky air ducts which pull humidity from the attic or crawlspace, or a malfunctioning AC unit.
So what should you do if mold is growing around the register? Fix the cause, from the above paragraph, then wipe it off. Don’t you need some kind of special mildewcide to kill fungus, like bleach, or a strong fungicide? It won’t hurt anything, but if you take away the moisture from condensation, mold cannot grow. You may have to remove the register to wipe inside the entrance to the vent, and you may have to paint some of the drywall with a stain killer to fix the stains around the vent cover. Be sure to wear protective clothing and a protective mask or respirator, and don’t try this if you have allergies or health risks.
Should you clean the air ducts? Probably, because most air ducts need cleaning, especially if they have never been cleaned. But not necessarily because of the mold growing around the register. How can you really know if you need air duct cleaning?