It's just months of neglect. We've had this account for 6 months and when we got it the coil looked like this. We put our filters in over this filth. The customer didn't want to pay to clean the coils so I cleaned this one unit and am going to show them that in a year it will still look just like new as long as we keep our filters in it.
Hopefully they will see the difference and have us clean all their units this winter.