Why is there mold coming from my Air Conditioner grills?

It's probably actually mildew. All conditions in an air conditioning system are right to grow this black, yucky mildew and all air conditioning systems have some degree of mildew in them. When the mildew starts to come out of the vents and get on your walls/ceilings you can have your ducts cleaned to help with this issue, keeping in mind it's not the last you'll see of it. Sometimes the biggest problem is having too-short duct runs. We've slowed down the mildew-on-the-ceiling problem for some of our customers by making the duct runs longer.   Adding Ultra-violet lights to your supply and return air plenum can also help with this issue.