Same issue here. I have a living room 30year old GE thru-wall-unit made in Japan that still works! Came with my condo and for the past 3 summers it has spit pepper size black particles onto the floor that are soft and easily smear when I clean them almost daily. I took out this very heavy until in 2019 and throughly cleaned it with paper towels and vacuum and it was disgustingly dirty and rusty. Less particles spit out but it did not stop. I live in Chicago area so I cover the until both inside and outside to stop the cold air from coming in so I'm wondering if this would build mold, however my bedroom a/c does not have this problem. I really miss the mini-split a/c units we had in Europe, seemed cleaner and more efficient system. I'm thinking of purchasing a new unit this summer as as I jokingly googled it but seems I'm not crazy now but even more disturbed. Cancer survivor here so its scary to think it could be mold. Good luck to you all in resolving this issue!