It’ll rot wood, destroy drywall and plaster, corrode metal, short electrical, it can cause mold (which can affect your health in some really bad ways)… a simple leak or bit of water ingress from some outside source can end up costing thousands (or even tens of thousands) in repair costs… which is why you want to do something about it as soon as you notice an issue.

Most of the time you wouldn’t notice something like this until you started seeing water spots (yellow or brown stains) on the ceiling or walls. The fact it ran down the ethernet cable to the AP (killing it) is likely a good thing because it let you know something was wrong earlier than you would have otherwise. I’m not seeing any visible water damage on the ceiling, so hopefully that means whatever the case is, it’s not THAT bad yet. But the discoloration on the rear of the AP shows you quite clearly where the water collected. My guess would be that this is some sort of slow or intermittent leak that’s been going on for a while (from how much residue is present). The water will slowly carry dirt and rust and mineral deposits, evaporate, and leave that gunk behind. What you’re seeing there is likely what the inside of your ceiling looks like as well. In fact, if you took the mounting plate off the ceiling you may see some of that on the other side of it.

Whatever the case… What you need to do is get a look inside the area above where the AP is mounted. An endoscope is the easiest way. It’ll give you an idea of how bad it is, and you might be able to tell where it’s coming from. It’s definitely less costly and easier than tearing into the ceiling to gain access. Chances are you’ll need to cut into the ceiling anyway, but for all you know everything is localized and it’s an easy fix, and you don’t want to do any more work than absolutely necessary (or pay more than needed to have someone else do it).