+1 for the red scouring pad method though only for non painted rims! I’ve used it with some neat g101 cleaner. Just need to make sure you rub in the direction of rotation* to avoid it just looking scratched randomly. I’ve found using any sort of polish on braking surfaces just gets stuck in any imperfections in the surface.
*as in round the rim, it obviously doesn’t make any difference which direction that is!
Is that actually brake dust or corrosion of the rim?
IME dust accumulates due to abrasion in wet conditions, and as such is essentially water-soluable. I've never encountered any that couldn't be got off with soapy water and a stuff brush.
Also potentially a bit odd to find it near the centre of the rim as the rotation of the wheel usually flings it outwards.. so perhaps this is something different..?
Wonder Wheels alloy wheel cleaner. It takes baked on brake dust off car alloys so it shouldn't be fazed by bike alloys. I'd test it on a small part first, just in case.