Slow Cooker Butter Garlic Asparagus and Mushrooms

1 pound asparagus, trimmed and cut into 2-inch pieces
8 ounces mushrooms, sliced
4 tablespoons butter, melted
4 cloves garlic, minced
1 teaspoon salt
1/2 teaspoon black pepper
1 lemon, zested and juiced
1 tablespoon fresh parsley, chopped