I am sure there are many technical terms to refer to different elements of a webpage. You are not necessarily asking about the "background" (that's the term that comes to my mind) of the page, although I did find several references to that under webpage elements and design. Sometimes the background can be "clickable" anyway.
I don't think it's a technical term, but perhaps the word you are looking for is "non-interactive". Any portion of a webpage can be described as "non-interactive" (background, images, text etc.), and clicking there elicits no action. I have seen that word used several times when referring to webpage content.
https://www.thefreedictionary.com/noninteractive
https://www.quora.com/How-do-we-differentiate-between-interactive-and-non-interactive-websites