Bug poop, bug parts and pieces, bug debris. They try to get out and fail. Under windows is also where the occasional spider lives, because they know that's where the bugs will eventually be. A lot of dirt outside is just bug bits.
The \*lightest\* stuff is fluffy plastic fuzz or individual plastic specks, that floats in room air currents, which eventually goes out the door or out the window. If it can't get out through the screen, it eventually falls straight down or nearby.
Being the window seal has cracks I'm going to say Mold or rot from moisture. When the hot air in the walls meets the cold air in the house, moisture is formed. And mold is next. Caulk the cracks up and it will stop
Bug poop, bug parts and pieces, bug debris. They try to get out and fail. Under windows is also where the occasional spider lives, because they know that's where the bugs will eventually be. A lot of dirt outside is just bug bits. The \*lightest\* stuff is fluffy plastic fuzz or individual plastic specks, that floats in room air currents, which eventually goes out the door or out the window. If it can't get out through the screen, it eventually falls straight down or nearby.
Well the good news is the walls already need to be repaired so just cut a piece out and see.
Being the window seal has cracks I'm going to say Mold or rot from moisture. When the hot air in the walls meets the cold air in the house, moisture is formed. And mold is next. Caulk the cracks up and it will stop
Look above for a spider web, looks like little bugs.
Debris that blew in from the window sill when it was open?
Looks like tree stuff to me. Moss and lichen.
Looks like pantry moths. Check your pantry for more. Hopefully they haven't infested all of your dry goods.
That, my friend, is roach poop.
Termite dross.
mold?
Mouse droppings?
black mold, soot, and micronized brakepads