I cannot make it to the store.
Here's a recipe for bechamel sauce. https://food52.com/hotline...
Alfredo is already a cream sauce with cheese. It's a very poor match with shrimp.
I would toss out the alfredo sauce for starters. Make a bechamel with some flour, butter and the half and half. Sauté the mushrooms and when they have thrown off all their water and are browned, add the shrimp for about 3 minutes. Then add the shrimpy mushrooms to the bechamel. Season to taste with salt and pepper. Voila!