Every Windows 10 Mobile Insider build comes with its own share of new issues. That's just to be expected with any kind of beta program. Windows 10 Mobile build 10581 is no different, and fortunately an issue with the Start screen getting funky is easy to resolve.
Here's the problem's official description:
If you restore a backup from a phone that had a different resolution, you may end up with a corrupted Start experience. The workaround is to go to Action Center (which won’t be corrupted) then Settings > Personalization > Start and apply a different background picture.
The fix is contained right there in the description, but Microsoft also posted the fix over at the Microsoft Community. You'll know you have the issue if your Start screen looks like this:
So, no big deal, really. Just swipe down from the top, go to Settings, and get your fix on.