europa - Not to worry. It simply means the lovely silicone grease is now wearing out.
Undo the nut carefully, as the ball valve is pushed against the tap with a spring.
Dry the ball valve thoroughly
Apply a thin layer of molycote 111 or other food grade silicone grease on its surface;
Apply a little bit on the o-ring inside the nut where the ball valve rests.
Carefully push it back in, straight and level, making sure you do not cross-thread!