If an apple/IOS device like an iPad or Macbook is used to open the app it will look for icons and use them if available. You could provide them for convenience but that is not needed.
So nothing to worry about.
I think that somewhere in your styling, this icon is addressed. It looks for it in your styling folder but you dont have it there. Just try to find out in the details of the log message in which folder it looks for it, and place the icon there if you want it, or search in the styling where it is addressed.
I just checked, funny thing is that we have the same issue in our app.
2017-09-29 06:16:12.986 Connector 404 - file not found for file: apple-touch-icon.png
2017-09-29 06:16:12.955 Connector 404 - file not found for file: apple-touch-icon-precomposed.png
2017-09-28 20:57:23.831 Connector 404 - file not found for file: apple-touch-icon.png
2017-09-28 20:57:23.815 Connector 404 - file not found for file: apple-touch-icon-precomposed.png
I will ask around about this and let you know when we solve this :)
We see the same 404 errors on a regular basis. The forum post below seems to address how to solve it.
https://forum.mendix.com/link/questions/87102