Looks like ash to me too. I have used it to smoke a rolled roast in years past, and with excellent results also. Smoked it on a sheet iron folded "Pit". Bent in a U shape and put a grill and another piece of tin on top to hold in the smoke. Smoked it for about 3 hours, low and slow.
UDS, cheap o grill, rusted chimney, yeller wannabe thermo pen, Maverick ET 732, 1950 model wife.