Here are a couple of quick videos K did to show you how to debeard mussels and peel shrimp.
Most mussels have what is commonly called a “beard.” The beard is made of fibers called byssal threads that emerge from the mussel’s shell near the bottom (or the “hinge”) and are what they use to hold onto rocks in the water. It’s best to buy mussels with the beards still attached because removing them causes the mussel to die.
The main thing you are trying to get out of shrimp when you clean them is the small black tubule running down the back. This is actually the digestive tract. Eeew! So definitely try and get all or as much of that out as you can. You can also often times buy cleaned and already deveined shrimp, but where’s the fun in that?!