Frass
Even though there’s nothing sinister about this picture, there’s something about the white substance that’s unsettling. The OP had cleaned up a pile of the stuff before, not thinking twice about it. After it reappeared, he decided to ask Reddit what it was and whether it was harmful.
On Reddit, someone explained that it was “frass,” which is defined as the refuse of perforated wood, usually as a result of insect activity. It was quite possible that a colony of termites was going HAM on the OP’s cupboards. If you see this much frass after a week of sweeping it away, you know you’re in trouble.