I would suggest cleaning the screen really well and also do a full reset. On the back of Nest Hub, press and hold both volume buttons together for about 10 seconds. Your Nest Hub will let you know that it's resetting.
I hope we were able to answer your question with the info provided by @Cathal_S (Thanks for your help). If you have additional questions, feel free to respond to this thread and we'd be happy to further assist you.