Hi CnR, yes, that area can get pretty dirty. It's mostly dust from the belt and rollers which lodges there as the mileage builds up.
If you have access to an air line give it a good blow out with that (but wheel it outside first so you don't inhale the dust - you know the score).
Otherwise brush as much of it out as possible with a stiff paint brush or something similar.
Then you can use a brake cleaning spray and wipe it as clean as possible.
Then stand back, admire your work, make a coffee and try to remember where you put all the bits!