The white stuff is possibly powder that's been rubbed off from dusted crickets/mealworms that got under the paper towels. Or! If it could be a piece of urate that somehow got crushed under there. (Ew.)
My geckos are vocal. I've heard all but one of them make some kind of vocalization, whether it's been a scream or hiss, a squeak, gurgle, yelp, battle cry, or whatever. I have a male that even barks at his food. When the cricket or mealworm stops moving right under his nose, he makes a "puh!" sound to startle it into moving again, and then he strikes. He's also the same one who likes to stand on the tallest hide and make a coughing sound. Judging by how tall and proud he is when he does it, it must be some kind of territorial cry.
The funniest is in the mornings, since I keep three adults in my room. I don't know who does it, but early in the morning when we're all sleeping, sometimes I'll hear these soft yelps that almost sound like puppy noises. They must do it in their sleep, because when I look over, none of them have moved or touched each other.
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chickabowwow

3.2.3 leopard geckos (Rosie, Locke, Lisa, Caesar, Tommy)