It's not dust. I've seen this on other TriAxis units and it's a normal thing, and it's purely cosmetic. The front panel appears to be a clear polycarbonate overlay (the front panel membrane switches are behind this overlay). The graphics are simply laminated to the polycarbonate behind the surface layer, and a lot of electronic devices use this type of material for interface panels (including some products I've designed).
For whatever reason, the graphic layer has either lost some of its adhesion to the clear layer over time, or the graphic layer has simply faded. The only thing I know for sure is that it's not due to direct sunlight, since mine was always in a rack, and indoors, and never facing a window.