This message is shown by your browser/OS when it thinks you're typing into a field. In this case, you're not typing but it looks like the browser/OS mistakenly interprets the taps and drags you do in the game as typing for some reason. Unfortunately, since it's part of your system,  we have no control over this warning and we cannot turn it off. 

This only happens in full-screen mode, so I would suggest getting out of fullscreen mode.