Why was the cooking liquor for my mussels too dilute?
Yes, the additional liquid came from the mussels and is going to enhance the flavor of your finished dish.  All you need to do is remove the mussels to a bowl, cover lightly to keep them warm, and then turn the heat up to high on your pan and reduce the liquid until it's concentrated before whisking in your butter. (I presume you added butter even though you didn't list it above?)  

If not...finish it by whisking in softened (not too soft, but slightly) butter (known as "mounting with butter"  monte au beurre) which will emulsify with the concentrated cooking liquid and add a luxurious texture and richness to your sauce.  Finish by adding your chopped parsley and adjust seasoning according to taste.