Grand piano soundboards tend to collect dust, and unfortunately they are pretty difficult to clean with any standard household equipment. However, there are some specialized tools, both commercial and handmade, that can do the trick!
Pictured here is a soundboard cleaning set that I carry everywhere with me (3 pieces). They’re not too expensive, if you hate dust and want to do it yourself on a regular basis. Alternatively, most piano technicians will do it for a nominal fee.
For annual customers who keep up with their tunings, I often do this service free of charge!