So if you don't use a custom phone layout, go to customizer -> mobile (smartphone) -> mobile spaces -> space between elements
if you use a custom phone layout, on the page switch to your custom phone layout and set the row gutter to 0
If you use an element grid or thumbnail grid, edit that element by double clicking on it.
On the left side there is a setting for spaces between elements (horizontal and vertical spaces). Set the phone vertical space to 0,
Ah ok I that answers your question from your email but I see now that your question here in the forum is a more specific one.
I'm not sure why there's a space right there.
in lay options -> custom css & html -> custom css for mobile enter:
.lay-content.nocustomphonegrid #grid .row.has-background{
margin-bottom: 0;
}
Have a great day!