Vent condensation or whatever this is

Does anyone know what causes this? This is the first time I'm seeing it in the ac vent for the kitchen.

https://preview.redd.it/4apas1cnb6tg1.jpg?width=4284&format=pjpg&auto=webp&s=2e58bf22bbcb58cf3fb205c1d4a505024e3360e0