What is this white residue collecting on my ac in...

Dust of some sort. Does it come from your cool mist humidifier or the feed or your apartment is the question. My place is very dusty and I need to clean my tents passive intake filters a lot. These screens captures a lot of it and my carbon filter pre-filter does not need attention. I use dry amends though and they are fine powders. The pre-filter will capture a lot of that and then it needs a cleaning after every run. In the winter when I grow, my cool mist humidifier makes a mess of aerosoled minerals, when its running I notice a fine white dust on everything from it. They don't talk about it a lot or when you purchase it, would have went with a wicking one if I knew better. If you use a hot humidifier "steam" or wicking style it does not happen as the minerals are not vaporized and they deposit into the wick. then you need to clean the wick or replce it but much easier then cleaning dust. Good Luck!

Are you using a humidifier? If so this could be caused by using hard water. If using tap water you can check your TDS. Where I'm from the water comes out of the tap at 440 PPM and if I use straight tap water in the humidifier i get the same white residue. If this is the case, you could invest in a RO system or just a good water filter to lower the PPM or buy filtered or distilled water from the store. If none of this applies then i would assume dust and you could try implementing an air filter for your tents air intake. I also use an AC Infinity tent and never use the small rectangular vent ports at the bottom. I buy some extra 6" ducting ( like the same used for your inline fan) and run out of the bottom ports. Then on the end of the ducting, outside of the tent, I zip tie a piece of air filter i cut out. 2 benefits of this is, only clean air comes into your tent and also the ducting with the filter on the end doesn't let light into the tent as it cant make it through the filter and the length of the ducting. I use to use the mesh vent ports but had some herm on me. Since switching no issues. Hope this helps! Happy Growing!!

It looks like cotton lint - small particles. It would be normal if you had a white towel or something similar inside the tent...

Atmospheric dust. Your pre-filter is doing its job.