Is it possible to easily highlight an area of screen and grey out the rest?

Hello Emmanual, 

You can use a combination of the solution that Sindy provided with effects that pause the timeline of the base layer to highlight a specific section of the video. One way to do this is to add the spot light effect in a separate layer, and then use the "Pause timeline of base layer" option of the layer to pause your video while the call out is displayed on screen.

Then, once the layer is closed, your video will resume playing. Here's an article which explains layer properties in more detail: 

Storyline 360: Working with Layers

And for your convenience, I've attached a demo file which demonstrates how to achieve the above effect. You'll want to capture a frame from one of your screen recordings that you wish to highlight, and then add this image file into the layer to create the highlight effect. 

Hope this helps!