I have prepared this recipe 3 or 4 times and it comes out great!
This recipe (is a take off, I believe, from Miguel's Famous Jalapeno White Sauce of Brigantine Restaurant Fame) is featured in Recipe Goldmine and is real keeper.
You may be able to adapt this one for your Mussels!
I would recommend that you remove the seeds and veins from the Jalapeno.
The cream and sour cream always always reduce the heat from chiles.
As a matter of fact, if you eat a very hot chile, dairy products like milk, cream, sour cream, etc will help greatly in removing the sting!