My iphone has "touch to return to call" at top of phone. How do I remove it?

Can you not touch the green bar and end the call?

If not and if a restart doesn't work as wjosten suggested, try resetting your iPhone by pressing the home and sleep buttons until you see the Apple logo, ignoring the slider. Takes about 5-15 secs of button holding and you won't lose any data or settings.

Hang up the call. If you're not on a call, first try turning your phone off, wait a few seconds, then turn it back on.