Liquamen is an ancient Roman fish sauce. For a modern substitute, use high-quality fish sauce (such as Vietnamese nuoc mam) or a mixture of anchovy paste and water.
Passum is a sweet wine made from dried grapes (raisins). You can substitute with a sweet dessert wine like Marsala, Port, or a reduction of grape must.
Saturei likely refers to savory herb (winter or summer savory). If unavailable, a mix of thyme and rosemary can provide a similar aromatic profile.
For best results, serve immediately. Mussels do not hold well for storage after cooking as they can become rubbery.