Maybe dried pollen that they are ejecting from the comb. Did you reuse some frames that still had some pollen stores in them?
I saw a lot of that when I reused some frames from a dead hive in the spring. Thought they would use the pollen stores but they just threw them out all over the landing board.
Leave it to me to think of worst case senario first, but that sounds an awful lot like wax moth larva droppings. If you haven't opened the hive up, you better do so and see if that is what it is. The sooner you catch it the better.
It's usually an indication of a failing hive. The bees remove them if their population is strong. They will become overwhelmed by wax moths if the bee population is crashing. Pull the affected frames in put them in the freezer to kill of the wax moths larva then reinstall the frame in a hive that's strong enough to defend itself. They'll clean out the webs and dead wax moth larva. You may need to shink the size of the hive to better fit the population. This may require that you remove a hive body or even put the hive in a nuc.
I lost 3 hives to wax moth larvae this spring in the 2 weeks between inspections. Saved a 4th by taking off honey super and top body then checking frames in last body. it did come back somewhat but may be lost before summer is over. If an established hive swarms it is vulnerable!
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