What adjustment can I make to fix this problem ?

I developed a new problem on my Trek 7300 Hybrid today. The right hand (rear) deraileur control seems to slip into higher gears with the most minimal touch. The gear changer is a twist grip. If I make the smallest movement it shifts into a higher gear. This began rather suddenly. The only thing different is that I washed and greased the bike just before the ride prior to this.

Can anyone tell me if there is an adjustment I can make to correct this ?

I have Attached photos of the derailleur & control.

Thanks.

Babel