Can you please:

In customize -> mobile -> mobile menu

enable "vertically center menu points"

Then, set menu point padding top and padding bottom to the same values like 30,
and padding left and right to like 20:

This will look weird in the preview, im fixing this with the next lay version, but on the website it will look correct.

Now about the rectangle you see.
This is actually this setting when you scroll down a little:

You can change the color there.
It is the background of the currently activated menupoint.

The rectangle that you see is actually just this: the background color of the currently activated menu point.
However, I see that your menu point top and bottom paddings are quite big, and they push the text outside of its menu point box. The active background color of the menu point looks displaced.

Probably you put this heading there to vertically center the menu. But I think using the setting for that and configuring it in the way I described above will do the trick.