Musty smell is hard to remove. If it was mine, I would get an old white sheet bigger then the top and lay it out on my clean garage cement floor. I would lay the quilt top face down on the sheet. I would sprinkle a couple of boxes of baking soda over the back of the quilt top and let it sit most of the day. I would then take the quilt top out in the yard and give it a shake to remove the baking soda and give it the sniff test.
I WOULD RECOMMEND THIS FIRST BECAUSE OLD QUILT TOPS CAN SOMETIMES BLEED WHEN YOU ADD WATER.
If still musty I might take the quilt top into the house and place it on the sheet in the bath tub, add more baking soda or retro clean and let it soak for a while. I would then let the water drain out of the tub, add more water to rinse and let the water drain out again. I would then use the sheet to roll up the quilt top and remove it from the tub. I would press out as much water as possible and line dry with the sheet to support it.