DIAGNOSIS The fabric cover is acoustically transparent but a dust magnet. You're right to be careful, as liquids or aggressive brushing can push dirt in or damage the material.
FIX The right tool for this job is a clean, dry, soft-bristled brush. A new makeup brush or a soft-bristle paint brush dedicated to cleaning is perfect. First, unplug the device. Gently brush the fabric in one direction—top to bottom—using light strokes. This lifts and removes the surface dust. For more stubborn particles, you can use the brush attachment on a vacuum cleaner. Hold the nozzle *just above* the fabric, don't let it touch and potentially snag, and run it over the surface.
IF THAT DIDN'T WORK For sticky spots or slight stains, *lightly* dampen a microfiber cloth with water—just barely moist, not wet—and gently dab (don't rub) the affected area. Let it air dry completely before plugging back in. Never use sprays, solvents, or soaps directly on the fabric.
HEADS UP Never submerge or run water over the device. The fabric isn't designed to be removed or washed separately on the 2nd Gen Hub. Forcing it off will break the internal clips.