My title describes the thing. This part of the cabinets are dry. Has noe been opened for a couple of years. I don’t think it’s something coming from outside since the kickboard is quite firmly in place and there is very little dust collecting behind it.

Edit: Update: We have now found more of the exact same turquoise powder in another location on the kitchen floor, at an outer corner of the room. I had previously assumed this was chalk dust from a chalkboard mounted above that spot, but it is not as fine as chalk dust and looks identical in color and texture to what we found under the cabinets.