I think that is the 0xff 0xff 0xff that the Nextion sends with its data. The serial monitor can't display it properly so you get that.
There's nothing to fix, you just have to handle it in your code.
I have a tutorial at the top of the display section, in there you will find my methods and, in case you prefer them, a link so another tutorial by Seithan, he does things in a completely different way to me. Pick whichever suits you best. I am pretty sure @Whandall also has his way of doing things too.