How do I clean dust build up on ficus tree?

A swifter duster might work.

I spray mine, either outside with hose spray attachment or inside in the bathtub

I wipe my leaves individually with a damp cloth. Yes. It does take a while!

Spray it off in shower or outside if not too cold.

On our large leafed plants, we spray them with water in the tub, then wet rag wipe them off to make sure to get the dust off, rinsing the rag frequently. Start at the top to prevent the dirty water from coming down on the lower already clean leaves. Leave in tub to let dry and for the water to drain from the plant from the water spray.

Melissa....Buy a can of air from an office store (Staples, etc). Works really good for me!