Can anyone tell me why my 3yo macbook keeps highlighting sections in green?

I would schedule an appointment at the Genius Bar at one of the Apple Stores, they can usually diagnois something within minutes and let you know how much it would cost to fix it.

In the “appearance” settings there will be a selection for highlight color, looking like this. Make sure it’s not set to green.

@ Deany I will definitely give it a try and see what is happening. It is very annoying.

A little more research has led me to believe it may be a mousepad problem. I have several pages of possible solutions to try this evening.