Is there something wrong with my central AC unit, it's wet?

I just went outside to water the lawn and noticed that the bottom of the AC unit is wet and is actually growing mildew. The AC unit is in the side of the house sitting on a big square slab of concrete to keep it level. The other AC unit (for the 2nd floor of the house) is dry as can be, but this one is just went. This can't be good? How bad is this? Should I call an HVAC person ASAP?

Answers:
it is perfectly normal for there to be some condensation around the out side compressor unit
it is possible that the drain for both the 1st and 2nd story units is located next to the larger of the two units, this is condensate and it occurs any time the humidity rises enough to match the dew point on the heat exchanger ( the part of the unit inside of the house that actually cools the house) this water has to be drained off to the outside and that is probably where it drains to. if the mildew is a problem, any good mildew cleaner can me used, or muratic acid to eliminate the stain
around the condenser.

Other answers:
yes,it would be smart to call someone out asap!those things cost to much to chance it!
yes,it would be smart to call someone out asap!those things cost to much to chance it!
Dirt , algie, fungus will build up in drain.Turn unit off and spray with garden hose until water drains freely from drain line.Clorine tablets will prevent clogs in the future.
Yes. It is low on refrigerant. The other unit is not. I'd have it check out by an ac technician.
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