cleaning wrx a/c coil?

What you are referring to is not the "coils" but the evaporator core inside the HVAC housing behind the dash. The mold builds up because of the standing water left in there after the AC is used and shut down, even though there is a drain some water will always be left behind and that promotes the growth of mold and mildew over time. Things you can do to reduce this problem include checking the drain to be sure it is clear and removing any clogs (when the AC is running, you should see water dripping under the car by the front passenger area, if you don't the drain may be clogged. Look under the passenger side dash for a rubber hose attached to the housing and if necessary disconnect that hose and snake or blow it out). Also the AC should be run in the "fresh air" mode as much as possible, use of the "recirculate" mode will promote more mold growth. When nearing your destination, turn off the AC at least a few blocks before you stop and park but leave the fan running, this will allow most of the residual water to drain out. Once mold has formed and is creating odors, it can be treated by spraying a disinfectant into the HVAC intake with the vent fan running (AC compressor should be off at this time). Depending on how bad the mold is, repeated treatments may be necessary. You can use regular Lysol, or get one of the specific products made for this purpose at a dealer or parts store. The spray can go in through the outside vents at the base of the windshield, or for more direct access to the evaporator intake you can remove the glovebox and open the door where the optional cabin filter goes, and spray directly into the blower fan. In fact if you do have a cabin filter installed it is best to remove it when doing this treatment.

I should also add that if you do have the cabin filter and it hasn't been changed in a long time, the filter itself can be the source of odors not to mention restricted airflow if it's really dirty. If you aren't sure whether you have one or not, definitely go in there and check. As I mentioned above it's behind the glovebox (9 screws to remove plus one little plastic screw/clip thing), once that's out you'll see the trap door behind which the filter resides. If there is nothing in that compartment you don't have one but you can always put one in (check parts stores and Ebay for cheaper alternatives to the $40 dealer ones). If there is a filter pull it out and inspect it, if it's nasty get rid of it and get a new one.