Place chicken on bottom of slow cooker.
Spread pesto evenly on top of chicken.
Squeeze the lemon on top of pesto.
Place butter on top.
Cover with lid and cook on low for about 6-8 hours, or high for about 4 hours.
When chicken is just about done, cook pasta according to package directions and drain water.
Remove lid from slow cooker and shred chicken.
Pour the cooked pasta and Parmesan cheese into chicken mixture and stir to combine.
Sprinkle the mozzarella cheese on top and place slow cooker lid on top and allow to sit for about 10 minutes, or until cheese has melted.
Remove lid and serve!