DIAGNOSIS That's hard water scale or mineral deposit film from your tap water, combined with soap residue. It's especially common if you use the 'Clean' cycle with tap water and then let it air dry.
FIX Don't use abrasive scrubbers—they'll scratch the plastic. Fill the jar about a quarter full with warm water. Add a good glug of white vinegar (about a half cup). Drop in a drop of dish soap. Run the blender on a medium speed for 30 seconds. Let this vinegar solution sit in the jar for 15-20 minutes. Empty it, then rinse thoroughly with clean water. The acidity dissolves the mineral film.
IF THAT DIDN'T WORK For stubborn film, use a citric acid solution (like a tablespoon of powdered citric acid or the juice of a fresh lemon) with warm water. Follow the same process. The mild acid is safe for the plastic and more effective on certain minerals.
HEADS UP To prevent this, after using the 'Clean' cycle or hand-washing, always do a final rinse with filtered or distilled water if you have very hard tap water. Then dry the jar immediately with a soft towel instead of letting it drip-dry. This stops minerals from depositing as the water evaporates.