Heat 1 tablespoon butter and 1 grated garlic clove in a small nonstick skillet over medium heat until foamy. Stir in the panko, season with salt and pepper and cook, stirring, until golden, about 5 minutes. Transfer to a small bowl and stir in the parsley.
Bring a large pot of salted water to a boil. Add the macaroni and cook as the label directs for al dente, adding the cauliflower during the last 4 minutes of cooking. Reserve 1/4 cup cooking water, then drain the macaroni and cauliflower; set aside. Carefully wipe out the pot.
Return the pot to the stove, add the remaining 3 tablespoons butter and melt over medium-high heat. Add the remaining 2 grated garlic cloves and cook 1 minute. Whisk in the flour and cook until golden, about 2 minutes. Whisk in the milk, return to a simmer and cook until thickened and smooth, 1 to 2 minutes. Whisk in the cream cheese and cheddar until smooth; season with 1/2 teaspoon salt.
Stir the spinach into the cheese sauce until wilted, then add the macaroni and cauliflower; season with salt and pepper. Stir in the reserved cooking water, 1 tablespoon at a time, until the sauce is creamy and smooth. Divide among bowls and top with the herbed panko.