I've read about screen burn and this is why the always on display moves around, but as the night clock doesn't move is there a risk of screen burn with this if used constantly?
no,if the developer has provided it,it must have been fool proofed and double-checked but i wouldn't trust any third party software,if the nightclock screen flickers even a bit,it may prevent screen burn,like there are charms or something.
I believe that the op is referring to the stock night clock feature of the edge screen. If so then that does move around on the screen to prevent burn in