I'd say they're mites. My BP did the same thing about a month ago. Just started sleeping in his water bowl and it drove me crazy cuz i didn't see signs of mites an he just came out of shed a week or two befor that. Come to figure out couple days after he started sleeping in his water bowl the mites showed up... and they showed up everywhere.
So my best bet is that its mites. Luckly your useing papertowel bedding already so its easy to track them.
Time to bleach the tank and get some PAM if you can. Good luck man. Hope it all works out.