if you are still checking for feedback, Penguin provided a useful resource and here is another on the US FDA (Food and Drug Adminstration) specifying labeling requirements for the most common food allegens.
If the item is a foodstuff, I might say CAUTION - Contains seafood (shellfish). DO NOT CONSUME IF YOU ARE ALLERGIC TO SEAFOOD (SHELLFISH). The list of ingredient should list Green Lip Mussels (seafood or shellfish - see link).
If a medicine or other preparation, then maybe DO NOT USE - CONTAINS SEAFOOD / SHELLFISH. OR DO NOT USE IF YOU ARE ALLERGIC TO SEAFOOD / SHELLFISH or HAVE ALLERGIES.
Too many suggestions here. Typically US food labels have wording CONTAINS: Seafood / shellfish / milk / wheat, etc.
But it seems like your intent is for a British market with different food labeling laws.