It's not an easy thing to get rid of in a harp case! If you're really determined, you can hand-wash the case with some mild detergent, maybe in your bath tub. I'd make sure to do it on a sunny weekend where you can leave the case outside for a day or two to dry thoroughly. It can take a long time for the foam to dry completely, and if it stays at all damp, it'll just get musty-smelling all over again.
I've read about other potential odor solutions, like spritzing with vinegar or sprinkling with baking soda and then vacuuming it up, but I don't actually know how well they might work.