Bring a large pot of salted water to a boil. Cook pasta according to package directions, and drain.
While pasta cooks, melt butter in a large skillet over medium-high heat. Add bell pepper, and saute until browning and tender, about 5 minutes. Add zucchini, and saute 2 minutes longer. (It’s OK that zucchini is still pretty raw at this point.)
Reduce heat to medium. Sprinkle flour over vegetables, and stir to coat. Slowly add milk about 1/2 c. at a time, stirring until sauce is smooth before adding more. When all milk is added, bring sauce to a simmer, stirring often.
Stir in mustard and cream cheese, and return to a simmer. Add shrimp, and simmer, stirring frequently, for 5 minutes or until shrimp are uniformly pink and C-shaped.
Turn off heat, and stir in hot, cooked pasta. Serve immediately.