I don't know about mussels... but I have a teensy bit of salt in with my snails and they could care less. I think the key is to dissolve it in water before you add it and to make sure that you add it SLOWLY!
We have a brackish fish tank and our snails are doing okay. But our Mystery snails did not do so great with salt in the brackish tank. Died within a week or two. I don't know about mussels though. Good luck!