Caulk/Drywall crack between Shower/Tub and Wall?
Get yourself some painter's caulk (not pure silicone--acrylic with silicone is ok). Cut the nozzle tip small at a 30 degree angle. Slide the tip flat along the crack and force caulk into the void. Gently scrape any squeeze-out off with a damp finger or bit of cardboard. Wipe any smears off the surface of your drywall with a lightly damp cloth. 

Wait a day and see how much shrink you get. Chances are you'll want to hit it again. Then paint if you didn't achieve a good color match.