Why does this happen? WARNING GROSS PICTURE

Everytime I make smoothies this happens. It's disgusting. I use whole milk or yoghurt and fresh vegetables and fruits, blend them in a new, clean, perfectly fine blender and fill a glass up and put the rest in the fridge for later. This picture is taken 30 minutes after it was made. This happens EVERY SINGLE TIME, no matter what I do. What am I doing wrong?!